Step 12: Attack!
Sometimes customers laugh, sometimes they curse, sometimes they are riding on the shopping cart when it locks up...
Sometimes people blame "magnetic" things in the area. They are probably the most correct.
Our personal favorite is to go on every full moon, or only when there's a thunderstorm. You could also make up a story about the supermarket being built over an Indian burial ground and tell them about it.
On the one hand, we feel sorry for the engineer who has to figure out why thunderstorms are setting off a byte-encoded trigger for the locking mechanism. On the other hand, HAHAHAHAHAHA!
Good luck! You'll need it!