Great game... Funny glitch.
Really enjoyed the game, fast paced fun with the standard array of weapons. Don't want to spoil anything, but the last weapon you unlock kind of ruins the game with it's insane power, but it was very satisfying seeing the zombies going down so easily by that point in the game, haha.
I wish there was more of a balance to the weapons. What happens is after you unlock a new weapon, the previous one becomes obsolete. My weapon of choice is always the shotgun and it was kind of lame to never use it past the 10th level or whenever it was. Still though, it was alwas exciting to unlock a new one and have at the zomgies.
As for the glitch, I came across this when trying to figure out how to change weapons f you die in a level (which would have been a great option by the way. The game would be unplayable if you accidentally walked into the next level with weak weapons). If you right click the background and choose "rewind" then right click and choose "play" to get back to the start of the game menu again and then play the game, the game doubles in speed... if you did it a second time, it quadruples, and a time after that it goes 8x speed. Now playing the game in 8x speed definately gives it a new edge, haha!
Great game though, nice one!