I accidentally entered a negative number in the scale variable in the transform panel, and got an error message that won't go away.
If I click OK, it comes back again. I managed to get rid of it by pressing escape quickly and then X-ing out the negative scale value.
Error message: "Internal error code: (1380437316:71+4) attempt to index a nil value".
Yes, this is a known issue - any time I get this I roll back changes (usually in the History panel) until it gets fixed. Hopefully we can make this a little clearer / smarter in the future. Yeah, nil value basically means some number is messed up somewhere.