I made some changes in the IDE (e.g. moving components around), and then tried to ‘undo’ (with Cmd-Z) multiple times, quickly.
I got an error dialog
A stack with the same name as the one you are trying to load
is already open.
Before loading /Users/alextweedly/Library/Application Support/Livecode/Cache/tempstacks/undostack63.livecode,
what do you want to do with stack:
Layout 1