It seems to me that the most common overly ambitious project that programmers (esp. Comp. Sci. grads) try to tackle is building your own operating system. (Trying to create your own programming language + compiler is probably even more common but not nearly as ambitious.)

For those (like myself) foolish enough to try: aside from the sheer size, what are the biggest *gotcha*s or unexpected roadblocks you've encountered in trying to create your own OS from the ground up?

Edit: A great OS question: http://stackoverflow.com/questions/43180/how-to-get-started-in-operating-system-development/