I've gotten it more than once with just typed text before. Cut and paste tiny sections of text to see where the problem was. It was just a normal sentence with no /?| or anything else that might get interpreted improperly.
Try turning off the WYSIWYG text editor - I seem to remember experiencing this issue in the past when the forum refused to parse (for some unidentified reason) some strings of text it couldn't handle in WYSIWYG that the regular old text editor doesn't care about.
I switched some time back during the issues with something else and have never got a server error since.
I can generate it repeatably this morning "(all input energy is converted to heat in the space)" without the quotes in the garage heating thread. I have no idea why it vomits over there but posts fine here.
Hmm, thanks. I tried two different computers, neither could post that phrase in the garage insulation thread, but obviously it works in this thread. That's deeply disturbing. Computers should consistently misbehave.