When I worked in store, people would come in all the time asking for items that were out of stock, even if they checked on the website for our stock specifically.
There are a few things that come into play with these errors, sadly when people steal there's no way for the computer to know, unless we've done inventory (I think it's done once a year)
Another reason might be that it is in the back, yes, but in my store unless OPS was in, we weren't allowed in the stock room. So even if I wanted to help, I wasn't able to.
If the item was new, sometimes there's a store launch date, different from online. We weren't allowed to sell it beforehand.
And the other, systems are wrong, one time someone called re the KVD perfume, ... we didn't even sell it .. lol...
So, yes I agree sometimes SAs suck, but most times we really can't do much if it's not out.
I hope that clears it up!