B12

Various steroid users (Clemens, Palmeiro, Tejada, among others) have taken the stance that they were just shooting up Vitamin B12. This, of course, begs the question of why one would inject a substance when it is readily available in pill form and every food on earth these days is fortified with it. Basically, these athletes are calling us all gullible fools.
Of course, there is another option. These athletes actually were unaware of what they were doing. In this situation they were given steroids by a third party and told it was B12, they took it, felt that extra burst of energy and all the other benefits of steroids, and thought, "Hey, this B12 stuff is awesome! I should keep using it." This would make the users essentially innocent (sort of like the insanity plea), but it also would show how little they know about health and medicine.
Personally, I think there's no way anyone, mush less a world-class athlete, should put substances into their bodies without understanding ahead of time what they are doing and why they are doing it.