A person would use it to inform some-one else that they have used too many hashtags in a tweet.
Hashslags are often new to twitter, just having moved from facebook, and get over excited easily by the fact that they can now hashtag.
Person 1: #MeganFox is a #sexy #beast
Person 2: dude, what's with all the hash-tagging?
Person 1: #calmdown it's #twitter man! #becool #chillax
Person 2: *cough* hashslag
Person 1: ¬_¬
Person 2: ...
-you use too many hashtags. #hashslag