The 2 that come to mind right now are Halo 3 related. Before the game came out Bungie was doing a special contest where you could win 1 of 1000 copies a week earlier then the game released. It was a pain in the ass and involved being in the right place at the right time and solving some puzzles. Well I managed to do it and be one of 1000 people in the world to get Halo 3 early.
The other one was a week or 2 after the game came out me and my friend spent 3 days beating the game on normal, getting all the skulls and terminals, beating the game on legendary, and then finally getting every online achievement without cheating to do them. It took us 3 days of straight playing but we managed to do it. Those were some of the funnest 3 days I can remember in a long time.