This video demonstrates how an IR camera, of the type that can be bought for a reasonable price and attached to a smart-phone, can be used to capture a PIN that was previously entered on a PIN pad, by analyzing a thermal image of the pad after the fact. When the human finger presses a non-metallic button, it leaves a thermal residue that can be detected on a thermal image, even if taken many seconds later.

The video refers to the article: Heat of the Moment: Characterizing the Efficacy of Thermal Camera-Based Attacks, written in UC San-Diego.