I recently installed X-Wing Alliance on my laptop. Given that the game was released in 1999 for Windows 9X based systems, it runs rather buggily on my modern pc. Anyway, I recently completed all the objectives of a mission, pressed the hyperspace button to get back to the base, and then waited for the end of the mission to load. It never loaded -- I just sat there in an empty expanse of space. Knowing full well that I'd have to redo it, I quit the mission. To my surprise, the game decided that I had successfully completed the mission, and allowed me to advance to the next one.
Other than that, I can't say I've ever accidentally won at a game; generally speaking, if I win it's because I'm trying to win. [sub]Of course, trying to win doesn't preclude losing.[/sub]