I tried leaving a message then replying to a message and got the error:
[Alert icon] Error, try again
where [Alert Icon] is an big exclamation mark. I then removed a question mark and it allowed me to post. I tried the same comment again with the question mark and it didn't. Are there some really bizarre rules or antispam filters that prevent certain comments on YouTube? Perhaps it was a timing thing, my IP address is shared.
Best Answer
Perhaps you stumbled upon a weird bug in YouTube's antispam system. I know YouTube will definitely prevent you from inserting links into comments. Since I don't know the specific comment you posted it is hard to determine whether it was your comment itself that triggered the error, or just a mere glitch in the system. I'm going to assume it was the latter, and say that you should not experience this error often.
As an aside, I've been using YouTube for quite a few years now and I've only had a bug like this happen once or twice.