Think of errors I had probably missed before I started compilation. Just about any long process I think of the stuff I forgot to check before I initiated it. :-D
Other than that I get up, take a break, I deserve it.
The incremental builds are taking about 5 minutes, which is not really long enough for me to do any big work, but I usually look if there are any new issues in the queue, look at code reviews (not the ones I'm reviewing, but what others reviewed) or try to do code review for smaller chunks of code.