to prevent bad faith editing?
you can delete it and make it again
I would be in favor of an option (or even enforced) preview button like slashcode has, but I don't care enough to spend time on it. Actual editing after posting, I'm against. You say it, you stick by it. I don't rly care abt ur grammar, and I do care abt ppl editing their posts after they get called out on wrongness. If there ever is an edit button, there should also be a "see previous versions" button next to a label that shows edit times.
Proofread your posts if that bothers you. Hell, type them in a word processor first so it will catch errors.
If editing was a thing it should only be allowed before someone has replied to your post, once someone has replied to the post it should be unchangeable. Besides people can already delete their own posts.