I'm trying to post a comment that is 20 characters long, but "Comments must be at least 15 characters in length." Is this a bug?
This is likely due to the SE software trying to be smart and helpful for you. If you're leaving a comment under Undo's post, and it would be that post's first comment — or, more accurately, the first one not posted by Undo or yourself — the system will assume that the comment is directed at Undo. Once it makes that decision, it'll figure that it can safely strip the "@Undo" out of the text of your comment. Finally, after that text is gone, it will count the number of characters you're trying to submit.
Most of the time, this reduces clutter and improves readability, but in cases like yours, it can be a bit confusing.