We had a previous team member in Fulfillment at my store whose INF (mainly OPU) was always very low. Like he always beat out everyone else but was never much help in finding things when asked. The day he left for a better job, he confessed to a couple of us that he routinely substituted items he couldn't find for slightly better items. For example, if an order called for the Family-size package of Oreos and we were out, he would substitute the Party-size package (slightly bigger) so he wouldn't have to INF it. He claimed to have done this for over a year successfully because he never substituted more than one item per order and always gave the guest either more of something or a slightly better something than what they ordered. He claimed guests were always happy to get more than they paid for and when a couple of times a guest did bring back an order because it wasn't exact, he was able to explain it away with an, "Oops, my bad, grabbed the wrong one."
I've never had the guts to try this myself.