One of the best ones I have heard was a guy a couple of table down from me complaining that his AR was a POS and must have a bent barrel, as he could not hit anything with it, after a number of mags and a target that was free of holes, he was getting frustrated. I went over to see if I could help, well he was one that bought on a panic, the salesman said he needed a scope, and a .50 cal ammo can of ammo, then sent him on his way.
Turns out he knew nothing about his AR other than how to put a mag in and pull the charging handle, I ended up showing him how to open the rifle, pull the bolt carrier out and so on, we then moved the target in to 25 yrds and did a bore sighting and a rough scope setting, it was so far off that when I looked thru the bore, then the scope, the cross hairs were a good 12 inches off center at 25 yrds, in a few shots I had it hitting center with the scope and then adjusted the peep sights.
After 30 minutes, he was hitting at 100 yrds and was a happy camper, other than he had wasted all but 4 mags of ammo by not knowing what he was doing, nice thing is, as he was getting ready to leave, I asked if he reloaded, he didn't so I offered to pick up his brass, he not only picked up all his brass, he put it in the ammo can and then left it on my table.