At the time we were working on a new "feature phone" with a 160x120 pixel display in 4 shades of grey, which was a huge upgrade compared to our previous models. Another feature was full screen images for the various applications, and we'd been implementing is into the software and testing it for weeks without problem. After the development cycle came to an end, our release team created a new software release and sent it to our test department, which almost instantly reported graphical errors back to us. We tested the software image on our own handsets, and half the screen was "garbage".
We spent weeks looking over the merged code, as well as debugging pointers code and found nothing. It wasn't until we were stepping through the paint code with a Lauterbach debugger that we noticed something was "off" with a pointer.
The platform was a 16 bit platform and memory was addressed using a page pointer pointing to a 64kb memory page. When we traversed the bitmaps fate would have it that this particular bitmap, in this particular build, was split between two pages, and when incrementing a pointer beyond the page limit, instead of incrementing the page pointer, it simply just overflowed and started from the beginning of the current page.
Another interesting bug we chased in the compiler was it's inability to add more than 2 variables in initial assignment.
i.e.
int a = 1+2; // a = 3
int b = 1+2+3; // b = 3
int c = 10 + a + b; // c = 13