Not unusual at all when the bike-shop employee doesn't use a torque-wrench and get that bolt to the proper specs. Without an automotive-style 3/8" ratchet wrench, there's no way anyone's gonna get a crankarm-bolt up to the proper torque of around 27-35 lb*ft.
Unfortunately in your case, the mating teeth between the crankarm and bottom-bracket spindle has been chewed up by the looseness and wobbling. This is invariably a rapid downward spiral where the wiggling motion it allows will cause even faster loosening and falling off crankarms. Keep an eye on it, if it falls off again, you'll most likely need a new crankarm.
Last edited by DannoXYZ; 06-24-07 at 02:14 PM.