This question already has an answer here:
- Annoying comment bug in review queue 1 answer
- Open a Review Item (checked for Close queue @ SO)
- Add a comment to the post being reviewed
- Edit the comment => "An error occurred during comment submission" immediately
UPDATE: Despite having identified the case with audits in https://meta.stackexchange.com/a/270353/174091, I still encounter this sometimes in normal reviews. The cause is the same: real comment fns are being replaced by fakes. In particular, I see this happening when I browse back to the review that I've just voted on.