I posted a long answer on Facebook to a friend's question. I got an error message of a big robotic smiling face. Is there any way I can retrieve the answer? I went into a lot of detail and don't want to recompose my answer.
Case-1: If you haven't reloaded the page already & Facebook still shows the original comment along with the error, like the following:
in that case you can still copy the comment, reload the page and then paste the comment in that post again.
Case-2: If the above is not possible (i.e. Facebook is not showing the comment at all or the page is already reloaded), then there is a possibility that error happened after your comment was successfully posted. In that rare case (perhaps your friend accidentally deleted the post or something like that), it may be possible to get the comment from your friend's email notification (if s/he has comment email notification activated). Ask him/her if s/he got the email notification with full text of your answer. With any luck, you may be able to retrieve your comment that way.
I got back a few accidentally deleted comments this way before, so I know it works sometimes.
Case-3: If neither case-1, nor case-2 works for you, then you are out of luck. Most likely that comment is lost forever. Unless you know someone from NSA :)
You may try asking for Facebook support, but I don't think they'll be able to help. Still, you can ask if it is that important, the worst they can say is "no".