I had this happen a while back, though I didn't have to cancel vacation. I was working on a C based MS-DOS based POS system, adding a feature to allow employees to check their timesheet from the terminal. I added the function and went through two weeks of testing with helpdesk and store personnel - it was flawless. Or so I thought. Approximately 2 1/2 months later, on a monday morning, our helpdesk was flooded with calls. Whenever someone went to print a timecard, the system locked up and rebooted. It was consistent, and even in our labs (and on my dev box), the error was there. I tried setting the system clock back to the date of the previous test, and the error was there. After two weeks of hunting, I traced the error to a function which was reading some memory after it had been free()'d, in a library I was using. Why the symptoms of this error had chosen to crop up two months after release, I don't know...