I think you hit it on the head with the second part of your post. As the complexity of games increase, so does the potential for glitches. Imagine the amount of testing that has to go into a game like God of War III on the PS3 versus the amount of testing that went into a game like Pitfall on the Atari 2600. There's obviously a lot more that can break with games that huge.
For the classic style web games I make with my chums, we usually only need 1-2 people testing them for glitches. For huge multi-million dollar games like GoW3, they need big teams of people constantly testing them, and even then, glitches still sneak on through as seen in my latest video:
BTW, the name's Rog, not Rob. :P