Went to replace the car battery yesterday.
Garage bod 1 looks under the bonnet, goes out back and gets battery off shelf, while bod 2 types the reg into the computer.
Bod 2: "We need to order it, it'll be in Monday, can you come back?"
The three of us look at the shiny new battery on the counter.
"Ah yes. The computer says we haven't got one, so I can't sell it to you. Come back Monday and we can fit it."
"Can you order it and fit it now?"
"No, the computer won't let me charge you until it arrives."
"But it has arrived - it's right there!"
"No, this battery has never arrived, according to the computer. We have a pallet of batteries that arrived a couple of months ago, but the computer says they didn't, so it won't let us send them back."
Happily, it turns out that when the computer says "no" it can be overridden with a handful of beer tokens.