Don't be mad at the computer. Had it not been so obstinate they would have just ordered the size you wanted then sent you on your way rather than expend the effort needed to find them. The computer didn't misplace them.
As far as the sandals, maybe they were closing them out. Not uncommon for retailers to put items on automatic markdown. Say 20% off for a week, then 30% off....
In which case the computer was right, it would have been the person responsible for the signing that was wrong.