I buy and sell on StubHub all the time, and unfortunately sometimes these things happen with tickets.

A few years ago, my Yankees season tickets were almost all sold on StubHub and they called me up to say they messed up and accidentally assigned me seats promised to 2 people. For my troubles they were keeping my same seats and moving 2 rows forward. Great except now I dont have the seats I sold. I had to individually go in and select to change the tickets and then the person had to accept it. Thankfully they all did because prices had gone down between sale and delivering tickets.

I agree getting the wrong tickets sucks, but unfortunately nothing StubHub can really do to prevent it.

You can minimize the risk by looking only for tickets that are available for instant download. If you only buy tickets with instant download, you avoid all of these issues, because the original persons tickets immediate deactivate and you get tickets with a different barcode.