When you attempt to pull an allnighter but do not manage to get your work done. This can be due to not staying up for the duration of the night or if you do not complete your work that the allnighter was intended for. Attempts to take a powernap are often times an early sign that a failnighter is on the horizon.
Avoid the crash coma at all costs by pacing out your caffeine intake. Often times speeders will be your best friend and will keep you going when caffeine would likely make you crash or you otherwise couldn't.
Tom and Luke had a failnighter last night when they did not finish their homework and spent the majority of their time coming up with a good definition for it on Urban Dictionary.