So what about things that are unstoppable in video games?
My first thought is of RPGs with unwinnable fights. You know, the bosses that are there for the storyline and your defeat is scripted one way or another? Your enemy is certainly unstoppable at that point. It tends to be a good way to indicate just how powerful the evil you're fighting is. Perversely, you must survive some of these fights until a certain event in the battle, while others regard your encounter as fulfilled even if you die before the enemy's ultimate attack.
What's even more fun to me though is the near-unstoppable boss, the boss that often feels too hard and whose defeat of the hero doesn't cause a game over, but progresses as if the loss were programmed anyway, but, with some planning, skill or just plain luck, you can actually beat them and sometimes get a prize (though usually it's just bragging rights). Almost as fun are the fights where you are expected to die, but sometimes you have to force the enemy's hand due to your preternatural ability to survive.
Of course, an unstoppable opponent, and thus inevitable defeat aren't relegated solely to RPGs. Plenty of other games feature unstoppable enemies, weapons, even environmental elements that force the player to rethink their strategy, or simply give up on some objectives. Although the definitive example of this is probably the game that can't be won.
But what's the fun to be had with an enemy that can't be beat, an objective that can't be accomplished, or a boulder that's inexplicably in your way? Indeed there are times where overpowering enemies just frustrate the player rather than inspire awe. But consider this: it isn't very often we encounter these super-enemies, these deities that can smite us at their leisure.
In fact, when you consider the premise of most games, the opposite is true. We, the player, are unstoppable. Think about it: we are given control of a character with a mission and, in most cases, we carry out that mission. No matter how much the forces of evil, or the corrupt government agency, or even the psychological manifestations of our own inner demons try to hold us back, we make it to the end.
To take this further, there are games where it's impossible to lose. VVVVVV is a game in which even when you die you just immediately return to the last (often nearby) checkpoint. There's a definite end point, and getting there is just a matter of sheer determination.
In another sense, our characters often become ludicrously unstoppable themselves. I mean, in Pokemon Diamond and Pearl you can catch God in a pokeball. In Shin Megami Tensei you face off with God as a final boss. In many forms. Over multiple games.
Unstoppable elements in games are all over the place. Frankly, there's no real definitive end to a game with a story if something in its universe isn't at least situationally unhindered. It's really a matter of execution that makes it cheap or exciting. So because I'm not unstoppable, I think I'll end this here. But feel free to post some thoughts. Make this the post that never stops!