My worst case of DRM? I'd have to say back when I bought Bioshock 1 off of Amazon. The installer was pure evil. What happened was it would get to 99%, then say it had to go online and download the Bioshock.exe. Guess what happened? It failed. Every single time. And to be extra cheeky, every time it failed to connect to the Bioshock servers (and my internet was not a problem, every other single website checked out), the installer would then DELETE the game folder.
So what I had to do was, when the installer said it would look the for the exe, I had to go to Task Manager, kill it, and download a crack. To play a game I legitimately bought.
Weird thing is, I never once had trouble playing a legit copy of Assassin's Creed II. I knew all about the DRM, and bought it fully expecting it to keep on kicking me back to the main menu, but not once did it do it. I didn't crack it, it was all legit.
EDIT: Escapist, why the hell did you warn comment number 4? All he said was that pirated versions of games are superior when cracked, which is more or less true, given that then we don't have to deal with DRM, just look at what I had to do with my legit copy of Bioshock.