When Zombies run into supermarkets/grocery store they glitch into the floor. Their legs are inside the floor, while their upper torso remains above the floor.
Can also recreate this as a player by vaulting into the supermarket. You can then act as the zombies do above, or even go prone to make yourelf invisible underneath the floor. You can still loot items, and pop up out of the floor to shoot/attack.
Real immersion breaker and potential annoyance here.