[stepurhan]

I think the phrase "no offense" much worse. No pun intended may be because someone has realised they've written something that looks like a bad pun but wasn't originally meant that way. It could thus be considered an acknowledgement that the pun was an accident rather than claiming it as a witty thought by the writer. An act of humility if you will (this sounds really clever but not because I was clever enough to think of it)

By contrast, "no offense", can only ever be used when someone has said something offensive. If you say something offensive by accident then apologise. If you know what you're saying is offensive up front then expect me to be offended despite your last minute weaseling.