I'll echo Blitzball, but also throw in Sphere Break from FFX-2.
I played X-2 during middle school, and Sphere Break is a game of math that really wasn't explained very well. Needless to say, I lost the Sphere Break tournament, which meant I had effectively lost my only chance at the Lady Luck dressphere. You can challenge the person you went up against again in a later Chapter, but it's a more difficult match. If you can't win either of these matches, you cannot gain the Lady Luck dressphere, which means no 100% for you.
It takes place on a 4x4 board with a core in the very center of the board, the number on the core is the "Core Number", then there are 4 "Entry Coins" which you pay up that surround the core, and those are surrounded by 12 "Border Coins". Each round you have to select one of the Entry Coins first, then you can choose any other coin. If the combined sum of the coins matches one of the Break Multiples (Multiples of the Core Number you must strive for) you achieve a Core Break and that round ends, if not, you keep selecting coins until the sum reaches one of the Break Multiples. To win you have to use up a set quota of Border Coins within a set number of rounds and within the time limit. If you lose, you lose the Entry Coins you put in play.
The real fucking kicker is that the final opponent of the tournament is one of your own crew-mates, who will refuse to hand over the sphere if you lose to him. Even though it's a fucking dressphere and he can't/doesn't use them.