I get this in my store too. At first, I thought that the register was just going offline mid-transaction. But, that just doesn't make sense to me, because when the register ACTUALLY goes offline, you can't do anything that would require the use of a card. When we get this message, we're still able to complete the return and put money back to a card without issue.
Also, the items that prompt this "Register Offline" message...if you try to return them again in a new transaction, they end up not being on the guests card anyway.
The conclusion I've come to is that it must be some sort of generic error message, more or less. Example, have you ever seen something on your register that says something along the lines of, "Card has no value?" I can't remember what the message says exactly, but sometimes our registers will briefly flash this message in-between processes. Like, imagine where a loading screen would be if we had loading screens. Well, I think the "register offline" message is some sort of generic message that pops up for when an item is not on a guest's receipt or card. Like instead of saying "item not found" some glitch occurs that forces us to finish the return, and the register is just programed to show that specific message, regardless if that's actually what's happening.
Again, it's just what I'm assuming at this point, because once you try to do a second return, the items end up not being on the guest's card anyway, which would be too odd of a coincidence. So is the register really offline? No, probably not. Is it just fucking up for some fucking reason? Yep. Lol. Hoping someone whom actually knows what this means...can tell me if my assumptions are close to correct or not.
