The phrase "no pun intended" is often added after someone made a pun or something that could be considered a pun. If this should be taken literally (i.e. it really was unintentional), then I'm not sure what exactly the reasons are for adding this phrase or if you should even add it at all.
I guess the intention could be to avoid that the sentence is viewed as a joke because of the pun. This seems reasonable when used verbally. You realise what you just said was a pun and try to clear it up. When written, it seems unnecessary though. The person could just rephrase the sentence and get rid of the pun if it isn't appropriate or too confusing in the context.
At other times it seems that the purpose of the phrase is to get the readers attention that you just made a pun. If that's the case why does it seem to be so well accepted? Isn't it the same as saying "hey guys, I made a joke, please laugh!?", which would be considered lame? I believe it would be more witty to just let it pass and let the more clever readers figure it out on their own. Some people are even more direct and just add a sarcastic interjection like "(ha!)" for obviously intended but bad puns.
So it seems there are multiple reasons to use "no pun intended". What is the general reception of the phrase, should I always assume it was intentional and how do I tell if it really wasn't? Should I use it if I want to make a pun or only to prevent confusion when there is a double meaning?