I don't think he did anything wrong.
I think OP was wrong to get in line when she wasn't done shopping.
It's not true that she wasn't done shopping. She was! She had her groceries unloaded and ready to be checked out when she noticed that she got one wrong item.
Do you think she should have loaded all her groceries back into her cart just to go replace that one item?
To answer the bolded question, I think she should have just gone ahead and paid for the groceries she did want and then gone back for the missed item.
And yes, I've been in that situation many times. I realize I've forgotten something or gotten the wrong thing after I've put my groceries on the belt or counter
. And if I'm alone (which is usually) I will just complete the transaction as is and either go back for the item I need or just add it to the list for the next time I go shopping. I think it's fine to send someone back to retreive the item if you have a companion with you, as long as they can get back quickly enough to not cause delay in paying, but if you're alone, I don't think it's cool to leave your groceries unattended.
I agree with Snowdragon. Also, he had no way of knowing if and when you were returning.
So he should have waited until the cashier was done with the first transaction. I don't think it takes a lot of common sense to see a pile of groceries on the belt and assume they belong to someone. He could have at least ASKED. Instead, he just cut the line.
That's what I do, when I'm the guy.
I see that the stuff is there but not person; I assume that the person ran to get something else. If they're back before the cashier is ready for them, it's their spot. I'd never move their stuff.
If, however, they're not visible to me (if I even know who they are) by the time the cashier is ready for someone, I go ahead. Again, I don't really move their stuff, I just give mine directly to the cashier, using whatever space is available ahead of us.
I'm not going to wait for any *extra* time (i.e., I signed up for time for the lady who's currently being checked out and time for you to be checked out--but I didn't sign up to wait for you to get back from wherever you went), but I will not declare myself until it's clear that there would be a gap in the cashier's workflow.
And I don't move until the cashier is ready for me. I don't consider you to have abandoned your place in line until that point.
Not everybody "got that memo." And he may have a stronger sense of "you need to hold your place in line."
As for the guy, I agree with the bolded. Again, I've been in that situation multiple times. Groceries on the belt in front of me with no person attending them. 99% of the time, the person returns before their turn comes up and I just wait behind them as I would have had they been there the whole time. The other 1%, the cashier and I look at each other, figure out the other person has not returned yet, and mutually agree that it's now my turn.
But again, as I said early on in this thread, although I disagree with what both the OP and the guy did, I don't think either thing was particularly egregious. Both were mildly in the wrong but nothing to get in a twist about. Both were the sort of thing that, had I been involved, I would have forgotten about the whole thing about the time I was setting foot outside the store.