affects mac and win. intermittent until you get it, at which time you can't avoid it.
As per Max:
Steps to reproduce:
1. open TiApp XML editor and any other (e.g text file)
2. Switch back and forth between them to get error dialog.
results: !ENTRY org.eclipse.ui.workbench 4 2 2011-03-30 11:08:47.722 !MESSAGE Problems occurred when invoking code from plug-in: "org.eclipse.ui.workbench". !STACK 0 java.lang.IllegalArgumentException