Quick answer: Triggers auto-reset on consumption — if an unintended transition consumes the trigger first, your intended one starves. Disable AnyState transitions that read the trigger, set Interruption Source to None on conflicting transitions, and watch the Animator in Debug mode.

You set the “Attack” trigger from code. The animator should transition to the Attack state. Instead it transitions briefly into a fallback state and back. You set the trigger again; same result. The animator is consuming the trigger somewhere unintended.

How Triggers Work

A trigger is a one-shot Boolean. Setting it via animator.SetTrigger("Attack") raises it. The animator evaluates all outgoing transitions from the current state at each frame; if a transition’s conditions include “Attack” and they’re all true, that transition fires and the trigger is automatically lowered. Two transitions can read the same trigger, but only one consumes it — whichever fires first.

Cause 1: AnyState Transition Stealing the Trigger

You added an AnyState transition with “Attack” as a condition for a debug or fallback state. From any state, the AnyState transition fires first because its priority order in the transitions list happens to be higher than the named-state transition.

Fix: open the Animator, click the AnyState node, and either remove the “Attack” condition or move it below the intended transition in the transition list. Reorder by drag in the Inspector.

Cause 2: Multiple Transitions Reading the Same Trigger

Two transitions from state Locomotion both read “Attack” — one to a Light Attack, one to a Heavy Attack. The order matters. The animator evaluates transitions top to bottom; the first one whose conditions are met fires and consumes the trigger.

If you intend the choice to depend on another parameter (e.g., a “HeavyMod” bool), add that condition to the heavy transition. Make conditions mutually exclusive so only one transition can ever be valid at a time.

Cause 3: Trigger Set While Transitioning

If you SetTrigger during a transition (mid-blend), the animator may or may not honor it depending on Interruption Source:

If Interruption Source is Next State on a transition leaving Locomotion, and the destination state has a transition reading Attack, that destination transition can fire before the original transition completes — consuming your trigger before you ever reach Locomotion.

Set Interruption Source to None for guaranteed sequencing:

// In the transition’s Inspector:
// Interruption Source: None

Cause 4: Exit Time Blocking the Transition

The transition has Has Exit Time enabled. Even with the trigger raised, the animator won’t fire the transition until the current state reaches its exit time. Disable Has Exit Time for immediate trigger-driven transitions:

// Transition Inspector:
// Has Exit Time: unchecked
// Transition Duration: 0.1 (or whatever fits)

Debugging in the Animator Window

While in Play mode, open Animator on the GameObject. The parameters list updates live; you can see the trigger toggle to true and back to false. If it never goes true at all, the SetTrigger call isn’t reaching this Animator (wrong layer, wrong controller, prefab override, etc.). If it goes true and false within one frame, a transition is consuming it — look at which state highlights.

Verifying

Add a behaviour at the entry of the intended Attack state:

public class LogStateEnter : StateMachineBehaviour
{
    public override void OnStateEnter(Animator a, AnimatorStateInfo s, int l)
    {
        Debug.Log("Entered Attack at " + Time.frameCount);
    }
}

Set the trigger, watch for the log. Logged once: working. Not logged: trigger consumed elsewhere — trace through AnyState and transition lists.

Understanding the issue

Animation systems blend pose data over time. The blend math is straightforward; the timing isn't. State machines, transition curves, layer weights - each compounds with the others.

The specific bug described above is the kind that surfaces during integration rather than unit testing. It depends on a combination of factors: the asset configuration, the runtime state, the platform's specific behavior. In isolation, each piece looks correct; in combination, the bug emerges. This is why thorough integration testing - playing the actual game in realistic conditions - catches things that automated tests miss.

Why this happens

This bug class disproportionately affects late-stage development. The work to surface it is interactive testing in realistic conditions, which only really happens after the gameplay is in place and assets are populated. Catching it early requires deliberate testing of conditions that look unimportant.

At the engine level, the behavior comes from a deliberate design decision in Unity. The engine team chose a particular trade-off - usually performance versus convenience, or generality versus specificity - and that trade-off has consequences when you push against it. Understanding the trade-off is what turns 'this bug is mysterious' into 'this bug is the expected consequence of this design'.

Verifying the fix

For shipping games, the safest verification is a staged rollout. Apply the fix to 1% of players for 24 hours; watch the affected metric; expand if green. Skipping the staged rollout means the verification is the entire player base, which is too high a stakes for most fixes.

Reproducibility is the prerequisite for verification. If you can't reliably reproduce the bug pre-fix, you can't reliably verify it post-fix. Spend time getting a clean reproduction before you write any fix code. The fix is fast once you understand the reproduction; the reproduction is the slow part.

Variations to watch for

Related bug classes often share the same root cause. If you find yourself fixing this issue, look for cousins: similar symptoms in adjacent systems, the same data flow but a different value, or the same fix pattern in another module. The catalog of 'we've seen this before' becomes valuable institutional knowledge.

Adjacent bugs often share a root cause. After fixing the case you've found, spend an hour searching the codebase for similar patterns. What's the same call with different arguments? The same data flow with a different entity type? The same lifecycle issue in a sibling system? Each match is a candidate for the same fix, or a related fix that prevents future bugs of the same class.

In production

For shipping titles with a long support window, watch for this issue resurfacing after dependency updates. Engine upgrades, driver updates, OS releases - each one can resurface a bug class you thought you'd fixed because the underlying behavior changed slightly. Regression tests catch the obvious ones; player reports catch the rest.

When triaging a similar issue in production, prioritize gathering data over hypothesizing causes. A player report describes a symptom; what you need is a build SHA, a session timestamp, and ideally a screen recording or session replay. With those, the bug becomes tractable. Without them, you're guessing at hypothetical reproductions that may not match what the player actually hit.

Performance considerations

If this issue manifests under high load (many actors, many particles, many network connections), profile the post-fix code path with realistic counts. The original cost was a bug; the new cost is real work, and real work has a budget.

Diagnostic approach

The diagnostic tools available depend on your engine and platform. Use the engine's native profilers and debug overlays before reaching for external tools. The native tools have context that external tools lack - they know which subsystem owns the code, which assets are loaded, and what state the engine is in.

For Unity-specific diagnostics, the editor's profiler is the canonical starting point. Capture a representative frame with the symptom present; compare against a frame without the symptom; the diff often points directly at the cause. If the symptom is non-deterministic, capture multiple frames and look for the pattern - the cause is usually a state transition or a specific input value rather than a continuous effect.

Tooling and ecosystem

The tooling around this bug class matters as much as the fix itself. Good logging, accessible profilers, and clear error messages turn 30-minute investigations into 5-minute ones. If your project doesn't have visibility into this code path, the first fix should add the visibility - the second fix uses it.

Within Unity, the relevant diagnostic surfaces include the standard frame debugger, memory profiler, and engine-specific debug overlays. Each one shows a different facet of what's happening. The frame debugger reveals draw call ordering and state transitions; the memory profiler shows allocation patterns; the debug overlay reveals per-system state. Bugs that resist one tool usually surrender to another - the trick is knowing which tool to reach for first.

Edge cases and pitfalls

Platform-specific edge cases are worth enumerating explicitly. iOS handles backgrounding differently than Android; Windows handles focus changes differently than macOS. A fix that works on the development platform may not work on every target. Test on each shipping platform deliberately.

When writing a regression test for this fix, focus on the boundary conditions that surfaced the original bug. Tests that exercise the happy path catch obvious regressions; tests that exercise the boundary catch the subtler regressions that look like new bugs but are really the original returning. The latter are the tests that earn their keep over the long life of the project.

Team communication

Document the fix and its rationale in the commit message or attached engineering doc. Future engineers will encounter related issues; the rationale tells them whether your fix is reusable or specific to the case at hand. Without rationale, the fix gets reverted or copied incorrectly.

If this fix touches a system several engineers work in, a short writeup in the team's engineering channel helps. Not a full design doc - a paragraph explaining what was wrong, what's fixed, and what to watch for. Future engineers encountering similar symptoms will search for the fix; making it findable is a small investment that pays back later.

“A trigger is consumed by the first transition that wants it. Make sure that’s the transition you meant.”

AnyState transitions are convenient and cause more bugs than they solve. Prefer explicit per-state transitions.