Perhaps by forgetting to scan items stored on the bottom rack of your shopping cart.

They have that hand scanner tool, so you can scan larger objects without picking them up, and I often will forget I have smaller objects under the larger one I scanned.

Also, scanning 1 item while holding two of it in your hand.

Technically you can't, as being inadvertent means you lack the mens rea to commit theft. More colloquially it is used to describe any accidents in your favor with terms of paying for purchases. Forgetting to scan an item, accidentally having two items get stuck together and only scanning one, and similar incidents.

I imagine it happens all of the time with self checkout.

I got home to find a package of microfiber cleaning cloths in one of my bags once. I have absolutely no idea how they got there. But I guess I 'stole' them. Was going to bring them back to the store but realized that would probably be super weird so I just kept them.

I've also walked out of Lowes with a couple dollars in random hardware in my pocket before. Went in for some screws so didn't grab a cart, decided to pick up some other stuff since I was there, and stuffed them in my pocket temporarily. Forgot about them during checkout. I actually went back and paid for those though because I felt like such an idiot about it.

If you have kids it's really easy. You don't notice that they put an item in their stroller or inside another item that you're buying (like you're buying a cooler and they put a toy in the cooler).

Technically it's your kids stealing, but since you are responsible for their actions, it's on you.

When this happens to me I usually let them know the next time I'm at that store (or if I discover it in the parking lot I go back in). Sometimes they charge me for it, usually they just say thanks for being honest.

I've done it in the UK (three times) by failing to notice my contactless debit card has not been accepted and walking off with the newspaper/sweets etc. (I've gone back later to give them the money, but this is seems surprisingly difficult for a large store's systems to cope with.)

I've had occasions where the self checkout has had an issue after scanning something and the staff come over and login and do something and you carry on. It might only be when you check the receipt later at home that you realise that those carrots or whatever that caused problems were just voided off the bill/never scanned at all but were allowed to be bagged so actually you didn't pay for them.

Often the staff look like they are on autopilot and aren't reading anything and just going through their shift on muscle memory so I imagine this happens a lot. Is it stealing if you tried to pay but the staff did something on the machine and it didn't charge you? I don't know.