World of Warcraft, there was an iron wall that held my entire guild back, and that was Grobbulus. It was insane. We could clear every other wing of the raid without a single wipe, even managing the much more difficult Four Horsemen battle without any problems, but every time we wound up facing the slime daddy it was a night of wipes and frustration. When we finally took him down, the rest of the raid fell to us in a matter of an hour. For some reason, that one boss was just such a hurdle.
It could be a mission that you can't solo and can't get help with in City of Heroes. Maybe it's a raid boss in Lord of the Rings Online. Or maybe it's a certain episode in Star Trek Online in which a bug breaks the game and prevents you from completing your objectives. Whatever the reason, we all have something in our games that just refuses to let us past, content that we'd happily never do again if we could get past it the first time. So what's your iron wall?