What causes an ArgumentOutOfRangeException in Unity gameplay code?

An index or range argument fell outside the valid bounds of a List , string, or array-like API — off-by-one loops, indices from saved or networked data, or collections that shrank between the check and the use.

How do I catch this when it only happens for some players?

Capture it automatically from the player's device with the full stack trace, the device and OS, the build, and a breadcrumb trail of what they did. That turns a vague complaint into a specific, reproducible issue you can fix without owning the hardware it happens on.

How to Fix “ArgumentOutOfRangeException” in Unity List and String Code

Quick answer: Clamp and validate indices at the point of use, re-check bounds after any operation that can mutate the collection, and treat externally sourced indices as untrusted input.

The message is generic but the pattern is consistent: an index that was valid when computed is invalid when used. In games that gap comes from mutation between frames. Here is how to close it.

How to fix it

1. Validate at the use site

Check if (i >= 0 && i < list.Count) immediately before indexing, not three systems earlier — anything can mutate the list in between.

2. Watch the shrink-during-iteration pattern

Removing entries while other code holds indices (kill lists, inventory compaction) invalidates them; iterate backwards or mark-and-sweep after.

3. Distrust persisted indices

Save files and network messages carrying slot or entry indices meet differently sized collections after updates — clamp on load and discard invalid entries with a logged warning.

4. Log the values in the report

Include the index and the count in the captured error so a player report is immediately diagnosable (slot=12 count=8 tells the story at a glance).

Catching the ones you can't reproduce

The hardest version of this to fix is the one you can't reproduce — it only happens on a player's hardware, OS, driver, or save state, under conditions that simply aren't present on your machine. A report that says “it crashed” or “it froze” gives you nothing to act on, so the bug survives release after release while quietly costing you players.

Automatic error capture closes that gap. Each failure arrives with its full stack trace, the device and OS, the build number, and a breadcrumb trail of what the player did right before it broke, so even a failure you have never seen becomes a specific, reproducible issue. Fold identical failures into one signature ranked by how many players each hits, and your worklist sorts itself worst-first instead of arriving as a stream of vague complaints.

This is where a tool like Bugnet earns its place. Its SDK captures every Unity error automatically with the full stack trace plus device, OS, memory, build, and game-state context, folds duplicates into one grouped issue with an occurrence count, and ties each to the build it first appeared on — so you fix the problem that hurts the most players first and confirm it is gone when its signature disappears from the next release.

Reproduce it once with full context and the fix writes itself. The hunt is the expensive part.