It must have been surprised by a door slamming.

Understandably, I thought it was dead. I'd be dead if my spine was crushed to a millimetre's width. But the little lizard was still alive, and instinct told it to run away from me. Its hips were squashed, as if it had been run over by the world's smallest bicycle. As it dragged itself away, it only succeeded in separating its torso and entrails from the rest of its body. But it was still alive.

I couldn't kill it mercifully.