The ingredient that ruins everything: Isinglass
For any vegetarian reading this at the bar holding a British beer: you might want to put down that pint. Guinness, among other British brewers, uses trace amounts of fish bladder in the brewing process. And since Guinness is a perfect beverage that will likely never be changed, vegetarians will have to do without it. PETA comes to the rescue with a fairly comprehensive list of vegan-friendly beers.
The ingredient that ruins everything: Lard
There is nothing more American than apple pie and baseball, or eating an apple pie while watching baseball, or throwing an apple pie at a man in a baseball stadium and then running away, leaving him to wonder why someone aggressively threw an apple pie at him. Sadly for vegetarian Americans, apple pie (or any pie, possibly) that's made with a traditional recipe can have lard, which is used to make the delicious crust.