Twitter could be cracking down on the age-old practice of passing jokes around, which is to some one of the fundamental uses that made it so popular to begin with.

The social networking site has been deleting tweets on copyright grounds when people are passing them off as their own.

Twitter, like many companies that host content from users, has an entire system for handling claims of copyright infringement. Under the Digital Millennium Copyright Act (DMCA), Twitter is provided “safe harbor” from copyright claims so long as it does not try to protect infringing material.

Typically, claims concern embedded media like photos and videos, or they’re for tweets that link to other websites that are illegally hosting copyrighted material, like movies. It’s rarer for a DMCA request to involve the actual text of a 140-character tweet.