Thieves saw through bike-racks, cover the cuts with tape, wait for bikes to be locked to them

PC Mark McKay, a police officer in Camberwell, London, tweeted this warning to locals to beware of bike racks that thieves have sawn through and camouflaged with gaffer tape; once the bikes are locked up, the thieves return, remove the tape, and make off with the bikes.

It's basically the principle at work in this magic gimmick, on a macro-scale.