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Interstate '76 was a driving game that included a lot of fancy weapons on the cars. One level contained a funny, but annoying, mistake. The game told you that you had to find your way out of a closed area surrounded by a concrete wall. The "correct" solution was to find a hidden ramp, drive up it, and fly over the wall -- which landed you in a pit, but that was essential for the next part of the story. However, some clever players realized that they could drop a land mine near the wall, then drive towards it at speed. The explosion would blast the car into the air while forward momentum would carry it over the wall. If the car was sturdy enough, they'd land damaged but alive. They fulfilled the stated victory condition, but the game didn't recognize it, so the level never ended. The game was only testing for use of the ramp, not whether the car was outside the wall.
When you tell a player to do something, then check to see if he's done it, you have to test the thing you asked him to do, not just what you wanted him to do. In modern games with richly-simulated environments (e.g. the Grand Theft Auto games), there's a good chance the player will find a way to meet your victory condition that you never expected -- and he should get credit for it.
Continuing in the same theme, we come to...
Avoiding incorrect victory checks does not mean that you should nitpick the precise details. If the player performed some action that by its nature included the victory condition, he should get credit for that too. Andy Lundell explains:
It's bad enough when the mission objectives are illogical, but when you start punishing the player for making logical decisions, you've gone to far. You usually see this in FPS games or sometimes in the single-player parts of RTS games.
My favorite example is from Red Faction. There was a mission where you were told you had to destroy a particular computer on the space station. Once you got there you were told that you had to blow up the entire space station and run for the escape pods. So I, quite logically I thought, assumed that I could just blow up the space station and not worry about targeting the computer specifically. I blew up the space station, jumped in my escape pod and ... and ... the game glitched. We were supposed to blow up the computer then blow up the station. (They had no explanation for this duplication of effort.) Apparently the game couldn't handle the fact that the level ended without the computer being specifically blown up, so I just got dumped back to the main menu screen. All because I tried to do things intelligently instead of the stupid way the level designers wanted me to!
Here's a clue, level designers: if one victory condition (blowing up the station) naturally includes another one (blowing up the computer), there's no need to check the second one at all -- and doing so could get your Twinkies taken away.