I think something similar happened to me. I was working on a project, and while C2 was still running, I got this message:
<img src="http://micahjones.info/images/bug.PNG" border="0">
(Sorry it's an image. The message wouldn't let me copy/paste.)
I think I pressed "Ignore". I changed a couple more things, then I exited C2. When I tried to open the project again, I got that same message (I think . . . or at least one very similar). After that, I got this message:
<img src="http://micahjones.info/images/bug2.PNG" border="0">
So now I can't open the file at all. I saved everything as a single file, which might have been a mistake. I don't really understand much of the content in the message, but it seems like it's telling me that something is missing.
When I received the message the first time, I think I was adding values to a variable every time the player overlapped an area. The event worked, but when I pointed one aspect of the action to a different object (without recreating the whole event), I think that's when I got the message.
Anyway, I'm running Windows 7. Here's the cap: