What is the X in X-mas?
A subspecies of logic bomb that is triggered by reaching some preset time, either once or periodically. There are numerous legends about time bombs set up by programmers in their employers' machines, to go off if the programmer is fired or laid off and is not present to perform the appropriate suppressing action periodically.
Interestingly, the only such incident for which we have been pointed to documentary evidence took place in the Soviet Union in 1986! A disgruntled programmer at the Volga Automobile Plant (where the Fiat clones called Ladas were manufactured) planted a time bomb which, a week after he'd left on vacation, stopped the entire main assembly line for a day. The case attracted lots of attention in the Soviet Union because it was the first cracking case to make it to court there. The perpetrator got 3 years in jail.