Various things.
For one, I have to give people credit at their constant ingenuity of figuring out how to put items in the most possibly inconvenient and incorrect location.
People who would take down an entire row of purses, dump them on the ground and just leave them there to pick out one specific one of the identical purses from near the back. Purses do not have expiration dates.
Homeless people. No offense but they tend to cause a lot of problems. One time I was checking someone in a line full of people, and this random dude with a heavy accent is asking me where to get a belt or some stuff and this homeless guy walks in and sits down in the doorway and then begins bellowing as loud as possible announcing how he is at a moments notice going to leave. He is leaving and is going to leave, he claims. He wants to make sure everyone is aware he is goin to leave.
So one day this older woman starts bringing things to the counter. At first it looks like its just one of those, "oh one more thing" scenarios, but she kept leaving and coming and going. Eventually she was making trips back and forth from the furthest reaches of the store to gather this or that. We have carts, mind you, so its not like she was being forced to do this. As the line builds up, we have to eventually move her stuff into a cart for her. When she comes back she gets all indignant like we personally offended her and we are bad people.
A woman was once very picky about her bag. This was weird. We had several sizes of bags and she presented items to be bagged that would require a mediumish large bag. but out of the blue she pulled out a bunch more stuff. At this store we have a rule that we try to only use one bag, and have to charge an additional 25 cents per bag we didn't believe was necessary, as the store had on multiple occasions run out of bags. We were bag conservationists. Anyways I start putting things into this bag and she starts yelling at me not to bend the loving paper. The mother loving paper is supposed to be straight otherwise she might as well just throw the mother loving thing out. Shes not going to loving pay for it if I bend that mother loving paper. So i fill the bag and the mother loving paper is intact. The bag has a handle of some fly swatter or hanger or brush or something sticking out and she yells at me now that I gave her two bags worth of stuff. I need to give her a second bag for all this stuff im giving her. Well no, I say, to make a stand. No, I can give you a bigger bag, but i will have to charge you if you wanted two separate bags. Well forget it this is holding out a line heres your free loving bag bitch. She mumbles well next time ill just bring my own loving bags. Well maybe you should bitch. Next In Line Please :D
And so such things happen.
tl;dr = I am that angry shop keep.
This isn't to say I act angry mind you, I am always pleasant and cheerful and willing to help out and do my best for the customer :D