I find this very aggravating. A bad guy has a gun pulled on a character, and is about to pull the trigger on some innocent victim. But then, from some unexpected angle, a protagonist intervenes and shoots the bad guy.
A lion chases a character up a ledge, and is about to eat his face. But then, out of nowhere, a tree falls over on top of the beast.
An ominously slow spike-ceiling descends upon a protagonist. But in some distant power plant, a neglectful porn-surfing worker blows his load on the control panel, effectively causing a power outage and allowing the protagonist to escape, with just centimeters between him/her and the spikes.
It's such a fucking overused device, that it entirely kills any suspense that it was meant to deliver.
Any other over-used trope devices that piss you off?