This error message is cramping my style this afternoon. If I knew what it was talking about, maybe I could fix the problem!
It seems to come up when I use EditorUtility.InstantiatePrefab, but not always. I've used that function many times before today, and never saw that error before.
You'll probably need to be more specific, could you provide some code samples of where this problem occurs, or perhaps explain what you are trying to achieve?
By the sounds of it, it looks as if you're referencing the name of a prefab that can't be found for one reason or another...
I had similar issue as well on my Mac Pro quad os 10.6 Recreating script/prefab connection was my solution