What's your most frustrating example of leaving a character alive for no reason?

We see it often, and we probably shout "What, are we conserving ammo all of a sudden?" either in our heads or out loud. But what are your most frustrating examples of "You should have killed them when you had the chance" moment? Sometimes movies put a time constraint, like needing to escape (which is a bit of a cop out IMO since it takes a fraction of a second to pull the trigger), but other times, it's a cheap setup for retribution later.