When posting answers, I've been intermittently getting a message about invalid HTML being changed. Nothing appears to have been changed and if I retry to submit, I get the same message. This is even with plain text, no code, no images, or links.
I figured out that I could edit the raw HTML directly and submit the post (get rid of the span tags), but still a bit of a pain.
Not sure how I managed to get the rich text editor to add those span tags. Next time it happens, I'll try and nail down what I did while editing to make this reproducible.