I've had kbin show me a 500 once or twice, but it was because my ethernet cable had slipped the socket.
I've gotten more complex custom errors of indeterminate type, generally when posting, but typically the error is in error, as the post completes as expected.
@Anomander
This is the second error I described.
However, I was not trying to tell you what error you experienced, only offering my experience for comparison, or to add to the list of things you're hearing about.
Cheers and Good Luck