Well I wanted to see if as the tension went up, people would post more frequently and faster ...
It started slowly, and as people started to get frustrated with waiting for the 21th, they started posting.
I didn't take into consideration simultanious posts... I though there was going to be a stagnation on the 19th post :s...
It did end faster then expected, but it's okay as long as it was fun ^^.

Mmh, why do you assume frustration as the driving force between posting? From the wording of the posts, it seems that altruism is a better explanation (i.e., people willingly forfeiting one of their two chances in order to enable someone of the community to win the game).

Stagnation at the 19th post was not to be expected imho because when everyone has two shots, and when you (realistically) assume that several people who watched the thread had still two posts left, those people would fire both shots quickly in order to increase their chances. Since the first shots of these watchers were likely to bring the post count to 21 already, there was no reason to wait. This is a forum of gamers, you need to take into account that they are thinking strategically. :)

Yes, I underestimated the number of active people on the forum during a 30-minute interval, and thought that most of the active would have posted at least once by the time it got to 19 ^^

For the Next Giveaway, I'll try a different thing :-).