Have you been cutting and pasting objects? There is a bug fixed in the next build where if you cut the last instance of an object, then paste it, the object becomes corrupted.
Ah, so that's what does it. I occasionally have this problem too. I've just gotten into the habit of making sure my icons were all correct before saving.
And work3, it does cause problems down the line. Your object picker will just get more and more jumbled up the more objects you add. You can still pick objects by their name though. And last I checked, trying to edit the icon of a messed up object results in a crash.
It's best if you have an uncorrupted backup you can revert to.