Adding some hilarity. This happened just right now, about five minutes ago.
I played as dominating Chinese military juggernaught, and basically "won" the game long time ago, having Mechanized Infantry and Destroyers versus Musketmen and Caravels. Usually I tend to drop game long before gap becomes *that* wide (and yes, need to move difficulty up), but this time I just decided to play to victory, no matter how trivial that would be.
My huge, 5-civilization continent (Standard Continents map) was filled only by China, ex-capitals and allied city-states. There were three civilizations beside me: Egypt, America and Aztecs. Egypt was irrelevant: Ramesess decided he has the right to speak to me rudely, well I captured Thebes and razed three of his major cities, Memphis and Alexandria included. His spirit was completely broken and he basically was my "vassal" at this point. Aztecs and Americans dwelled on shaky continent, roughly reminding of the New World. At least Americans were at north, and Aztecs were at south.
When I got bored of skipping turns, I decided to capture their capitals and finish the game. My attacking force was very small: two Destroyers, two Mech Inf to Tenochtitlan and just one blitz-promoted Mech Inf (that was my starting Warrior, heh) to Washington. I thought about synchronizing attacks on capitals, but then decided that I just don't want to. "Conquistadors" got to Tenochtitlan first. And, well, they captured it.
I skipped some turns, moving embarked elite Mech Inf to American shore. Conquistador party destroyed weak Aztec defenders around Tenochtitlan, and captured some other city just because they could.
Then SUDDENLY I won. Because Monty the Mad decided it is a good idea to capture Washington, making my China the only one civilization which didn't lose its' capital. Basically, his kindness (and insanity) saved me three-four turns of travel.
Don't know about you, but it doesn't seem to me that AI "tried to win" at that point. He basically suicided himself in gameplay terms. Of course, being Montezuma and all, it actually made some kind of wicked sense, but... this is still wrong if concept of "AI playing for victory" is true. While in that particular case AI had no chance of winning at all, this situation may be repeated in other environment, where victor hadn't been so clearly decided yet.