I have too many games and not enough time to play them repeatedly. In fact, its very rare that I ever get in multiple playings of a game at one sitting - the exception being when I see my sister and we play a few face to face games of Blokus or Ingenious. At any rate, the problem this causes is that I often play games where I am either completely unfamiliar with the rules, or at least don't remember them all. Case in point - my latest playings of Descent. After the playings, I like to review the rules if the game is complex. In re-reading the rules I found that I had (again) forgotten to collect threat tokens (as the overlord) when surges are rolled (two surges = 1 threat token). Not only that, but the last two times we've played, we played with the monsters at an easier level than we should have (we used the card for the number of heroes, not number of players).
Descent is not alone - I always feel like I'm forgetting some rule. Sometimes, its an important rule, sometimes its some side case or a tie-breaking condition. How do some people keep all the rules straight?