I went to a pub recently and ordered a black and tan expecting Guinness and Bass, they made it with Guinness and Sierra Nevada Pale Ale. It was the worst black and tan I've ever had.
Sierra Nevada is terrible beer, even worse in a b & t. When I asked they said they were out of Bass. Make sure your b & t's are the real deal. On the plus side, I had a few Irish Car Bombs which are damn delicious. Guinness with Irish Whiskey and Irish Cream in a shotglass you drop into the Guinness right before you chug. Definitely a great drink.