Public service announcement: always check the rack or post to which you're locking your bike!
Whoever loosened the fasteners had planned ahead and left them finger-tight so the bike rack looked secure, and people locking their bikes would just think "Oh, the rack's just loose." This way, tools aren't required--just wait for someone to lock their bike, quickly unscrew the fasteners, lift the rack, and take the bike.
I showed the manager. "Who would do such a thing?" she asked.
"Bike thieves," I said.
This particular rack is at the Solano Avenue Safeway, in Albany, California.




(this is a repost, with better detail and explanation, and a G-rated title. Sorry!)