theft?

Yep.  Theft.

what if he made one purchase, didn't notice the error immediately, noticed 2 days later, and tries to go give it back, and walmart has closed down business forever.

Not theft. But I would think I would ask first.

I view this as intent. From the link the dude knew what he was doing was wrong - theft.

interesting.  so   where does it go from not theft to theft ?

going back for more?
noticing it immediately (define this!)   but not acting ?
what if he goes one day, doesn't notice, goes back the next day for something else, notices THEN, but had 1 moment before paid with the bill he shouldn't have received  to begin with .
?
what about walmart's policy not to prosecute for shoplifting under $25 ?

(not trying to be an *bunghole*, just asking seriously how law should be framed in instances like this)     grey areas rule !





for the record, if i didn't notice for 2-3 days  i'd probably pocket it .  if i noticed on the spot i'd admit it to the manager. 

To answer your question as well as address the other comments.  It wasn't theft the first time.  If he'd reported it and they let him keep it, whatever, no problem.  Going back nine more times was theft.  He knew what he was doing.

A friend of mine (well, not really, he's an asshat, I just know him) bought two Bombadier JetSkis many moons ago.  At the time they were about $4-5K.  He bought them on his VISA card.  Well, the bill never showed up.  He went back to the store and they said, "no worries, we were paid".  So then he reported it to VISA.  He got the usually runaround and after a half-dozen calls he was on with a supervisor.  She told him, "we can research it if you'd like but that's a $250 research fee."  So he thankfully declined and didn't look back. 

That's not theft.  The guy in the Walmart story is a crook.

This will be debatable I'm sure. If I pocket the money without looking at it (which I rarely do) and notice two days later I'd more than likely take it back. However I'm not sure at what dollar amount I'd make a special trip. $4 $6 $20?? 

Noticing it immediately I would definitely resolve it immediately. Noticing immediately and not acting is stealing IMO. You are not due the $20 no matter what the automated machine thinks.