I got the official confirmation while playing Left 4 Dead 2. So many zombies so little time.
This is my life, and it's ending one zombie at a time.
53,599 zombies killed tonight. What was the name of the achievement?
In Dead Rising, there was an achievement for killing 53,594 zombies (the real population of the town the game is set in)
In Left 4 Dead 1, Valve referenced this by "one-upping" Dead Rising by requiring 53,595 zombies to be killed.
Then, Dead Rising 2 had an achievement for 53,596
Then, another game had 53,597
Then another had 53,598
And now L4D2 has 53,599
And now I have that achievement.
Z-GENOCIDEST 2: EPISODE 2