Took me years to figure that out.

Wait, they still use incandescent bulbs? I thought museums switched everything to LEDs years ago for safety. That's wild.

Actually in Prague, the validation rule changed a few years back. For most city transport there, you now buy a timed ticket from a machine that's already active, so you don't stamp it. The paper tickets you need to stamp are pretty rare now. The real trap there is forgetting to start the timer on a mobile ticket in the app.