I've had one of those a couple years ago. Not sure what it's called. Just a hiccup in the reproductive tract and very disturbing to see. Don't think it's a long term issue.
Is she a new layer? Weird eggs happen often with new layers. Search weird egg on byc, I think you'll find something similar. But if it doesn't happen again or often it's still not a worry. Things just sometimes don't work right, lol.