I ask because I just recently tried to correct a post that had a command in it - but the command was wrong. The poster had typed chwon instead of chown. Of course this means it would not work. However Ask Ubuntu would not let me make an edit smaller than 6 characters.
I feel it is important to be able to alter small, but critical errors like this (without it, the new user may not get their problem solved).
Add an alternative or improve the answer. Especially relevant to command-line questions where things can be refactored to improve how they work. For example I've been going back to my answer for How can I backup my PPAs? and slowly cutting down its length. I'm sure there's an awk waiting to happen. Exciting times.
Look around the post for language errors. Capitalise "i", "ubuntu", etc. Remove thanks notices. There are few posts on the site that don't need at least 6 chars changing to improve them.
See if you can rephrase something without changing the meaning. Break big walls of text into nice paragraphs. Swap in synonyms.
Lastly, if you've got no other option, add some fluff. Something like an HTML comment as jokerdino points out.
If it's not important (eg if it's point #3 or point #4 above) ignore it, or find something more important in the post to fix.
Not that I recommend you or any others to do it, but if you can't seem to hit the minimum 6 character edit limit, add in a <!--> at the end of the post. It won't show up and will help you reach the 6 characters.