As Drage said, yes, there is a limit. It is not an arbitrary limit; it was placed there for a reason, though that was before I joined the site. While I agree that it is frustrating at times to not be able your own posts*, it's not impossible to ask the appropriate mod to do the tweaks for you**, and it's usually not necessary. It's especially cool to me that things are set this was, so the site can serve as a sort of static archive, with the added benefit that people can't get away with writing a post, then deleting it.
(*: yes, I am aware that moderation & administrative staff does not have the restriction.)
(**: I know I don't mind making genuine edits so long as I get a clear request, and I know the others are pretty much the same way.)