lead bubble

Something (such as a character or vehicle in a video game) that can take a certain amount of damage without any degradation of performance, until the point at which it runs out of hitpoints and suffers spontaneous existence failure. The idea is that it's like a bubble in the sense that when it's destroyed, it just suddenly "pops" out of existence, but it's made of lead, so it's stronger than an actual bubble. The name comes from Megaman 2, where one of the weapons Megaman could get was called the "Bubble Lead". Which was probably a reference to how it "leads" you and finds hidden pits, but a lot of people started pronouncing it like the metal lead...