A while ago, I was using an emulator for a 32-bit processor I was writing code for. To detect access of non-initialized storage, instead of having an out-of-band bit for each word, it stored hex deadbeef in each word before program start and, at each memory read, checked for this. If it was found, the enulator halted with an error.
This was very handy in catching errors. I never saw this particular word actually generated, so it never indicated a false error.