What causes a Unity collection-modified exception?

You added to or removed from a list, dictionary, or set while a foreach was iterating it, which invalidates the iterator and throws this exception on the next step.

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 “InvalidOperationException: Collection was modified” in Unity

Quick answer: Iterate over a copy when you need to modify, collect changes and apply them after the loop, or use a reverse for loop for removals.

“Collection was modified” means you changed a collection while looping over it. Not modifying it mid-iteration fixes it. Here is how.

How to fix it

1. Iterate a copy

If you must add or remove during iteration, loop over a copy (ToList or ToArray) and modify the original. The copy is not invalidated by changes to the source collection.

2. Defer the modifications

Collect the items to add or remove into a separate list during the foreach, then apply them after it completes. This keeps the iterated collection stable while you walk it.

3. Use a reverse for loop for removals

To remove items by index, loop from the end backward so removing an element does not shift the indices of items you have not yet visited, avoiding both the exception and skipped elements.

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.

The bug you can't reproduce isn't gone — it's just invisible until you capture it from the player's device.