Not sure if anyone else has seen this happen, but it's a cool and funny little easter egg that PopCap implemented. Basically what happens is a zombie will start chomping on a Wall-nut. Then, its cheeks puff out, its face turns blue, and it falls to the ground holding its throat. After a bit of twitching on the ground, it dies. It's funny as hell to see, but I've only encountered it twice, and I've logged in tons of hours. Also, I noticed it's only happened when I've inputted one or more of the codes, so maybe that has something to do with triggering it.
Just a cool little tidbit I'd thought I'd share.