hermes200 said:
Many times.
From the top of my head: the final stage of Braid. It was so cleverly constructed I felt like standing and clapping...
Seriously. This was well and truly amazing the first time I saw it. Can't say much more, lest I spoil it for anyone who hasn't had the pleasure of playing, but if you've been following the story at all, the final stage is a masterstroke.
Street Fighter III is an incredible example of brilliance from simplicity. The parry system is the most well-designed, perfectly-realized mechanic in the history of the genre.
Slightly off-topic, but I felt compelled to mention that gaming has a fairly storied history of accidental genius. The "cancelling" system of fighting games, as many of you probably know, was originally a glitch in Street Fighter II, but it worked so well as a mechanic that it basically redefined the genre. Similarly, Final Fantasy 3/6 and Super Metroid are tied, in my opinion, for the title of "accidentally the best game of all time." FF3/6 earned this distinction for being such a disgusting mass of bugs, glitches, and unintended consequences that allow the game to be played and experienced in a multitude of ways. You haven't truly played FF3/6 until you've beaten it without learning magic or gaining levels.
Super Metroid, on the other hand, probably introduced "sequence breaking" into the gaming parlance. So extreme is Samus' degree of mobility (again, resulting from a combination of well-designed mechanics like the Morph Ball, shinespark, and bomb- and wall-jumping, and a host of glitches like the instant stop, short charge, and speedball and mockball), that she is able to pretty much go anywhere she wants, whenever she wants. Bosses and mini-bosses can be beaten in any order, and a staggering number of items can still be acquired without ever upgrading her suit.
So, I guess what I'm trying to say with that little digression is that sometimes it's not (just) the designers who elevate a game to the level of "genius"--sometimes it's the players.