Reminds me of one of the more confusing bugs I've ever encountered in my life. I was throwing together a quick UI with Adobe Flex, and for some reason every time you clicked a particular button, the entire UI would shift 20 or so pixels to the right. I spent hours scratching my head until I noticed this for loop:
for (x=0;x<variable.length;x++) {
// blah
}
I wasn't declaring the x variable, so it was using the x part of the x/y positioning of the UI container. Woops.
edit: initialise/declare brain freeze