Dirty Fingers.

If a person asks you to step in, try to do the minimum to get them back in a known place. Describe what you’re doing, as you’re doing it — think of yourself as a tour guide, pointing out the sights. Ask for feedback to make sure your audience knows what you’re doing, and hand back control as soon as you can.

EFF Security Education Companion: A Word on Touching Other People’s Devices: Don’t.