This is especially true when games are very large or complex, and have a long list of possible situations. Even in 30K hours, the testers couldn't test every single possibility in the game. Did they test what happens if a car explodes while upside down on a rock at night in the rain while the player is in the middle of the bike-mounting animation? Did they test what happens if you're in the gps fleet tracking (https://www.intellitrac.com.au/)
middle of reloading a gun when a building collapses in mission 17 when your hand is supposed to cover your face, resulting in a weird animation glitch? Did they accelerate to mass speed while manipulating the camera with the mouse on its highest sensitivity but the aiming inverted? There are some bugs that only happen if you did one thing in one part of the game and another in a separate part 10 hours later -- if they were testing chunk by chunk (the usual situation), maybe they never saw that combination.
Werd.