When there's X amount of time left during the climax (usually anywhere from less than a minute to several minutes) until the really bad thing happens, and whoever is editing the movie lets it clearly and obviously go way past that amount of time before the situation resolves itself.
Like, I can suspend my disbelief a little bit. If the world end in 5 minutes and 6 mins of screen time happens before the day is saved, that's fine. But if it's something like 30 seconds or a minute left on the bomb timer and you let 2-3 minutes of screen time go by, then fukk you. You should've gotten a better editor or picked a longer time frame.