Actually, the intent of the OP does matter. Shoplifting is, however, like most crimes simply a general intent crime. What that means is that the defendant does not need to have had a specific intent to violate the law but only an intent to do the act that the person was accused of committing. In this case, the intent necessary would be to take the item from the store without paying for it. The OP need not know that doing so was a crime or have an intent to commit a crime (though in the case of theft pretty much every little kid is taught that stealing is wrong and that it is crime). The problem for the OP is this: there is a general presumption that one intends to do those things which he or she actually does. So when the evidence is that the OP took from the store an item that he or she did not pay for, the presumption is that the OP intended to do just that since people usually do not do things they do not intend to to. The OP would have to convince a jury that he or she did not intend to take it without paying for it, that there was some mistake of fact that led the OP to think the item had been paid for. That’s not easy to do without independent evidence to back it up. If the only evidence on that point is the OP’s own testimony that “I had no idea it didn't pick up the price” a jury is going to be rightfully very skeptical of that claim as pretty much every thief denies the crime and says he or she had no intent to take the item. The statement is self-serving and a jury won’t give it much, if any weight without something else to back it up.

