What causes duplicate instance IDs after merging GameMaker rooms?

Both branches added instances to the same room, and the merge produced duplicate instance GUIDs or layer IDs, so GameMaker either drops instances or throws on room load.

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 Duplicate Instance IDs After Merging GameMaker Rooms

Quick answer: Open the room .yy, give each duplicated instance and layer a unique GUID, ensure every instance is listed in its layer's instances array, then open the room in the editor to confirm.

After merging room changes, some placed instances disappear or the room fails to open. The room's JSON now contains duplicate IDs that GameMaker cannot disambiguate.

How to fix it

1. Find duplicated GUIDs

Search the room .yy for repeated instance and layer "name" GUID values. Each instance and layer must have a unique identifier across the whole room file.

2. Reassign and relink

Give one of each duplicate a fresh GUID and make sure it is referenced in exactly one layer's instances array. Orphaned instances not listed in a layer will not appear.

3. Verify in the room editor

Open the room in GMS2 after editing. The editor will normalize the file on save, so once it loads correctly, save and commit the cleaned room.

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 GameMaker 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 errors you never hear about are the ones quietly costing you players. Visibility turns them into a worklist.