Disclaimer, I seriously had to rant about this.
I work at a relatively large bike shop, and today I worked another busy Saturday.
As I had a driving lesson this morning, and dropped off I was without any way to get home and called my grandpa if he can pick me up, this part is important. After 4 PM I shut down the computers, closed the PIN terminal and flipped the signs. I did not lock the front door however to let my grandpa in. While I was waiting for my grandpa, me and my coworker decided to go to the shipping receiving area outside with a couple of beers.
After about 10 minutes of closing I went back to the store floor with a beer bottle in my hand to check if my grandpa already arrived, but instead I see a random man. Not really caring about the beer bottle I told him 'I'm sorry, but we're closed.' the guy replied with 'but the doors were open and I only need something (that we didn't have)!
I told him that's because I am waiting for somebody who does get to come in, and briefly checked what he needed. We didn't have it. Guy understood it and left, only for unlettered number 2 to arrive at the scene too, and ignore TWO 'closed' signs as well! Still having the bottle in my hand I told the guy we were closed down, said the doors were unlocked too. He only needed one thing but I couldn't do the transaction as I already shut everything down, and I'm not going to bother booting our POS computer again for one single item worth 10 bucks. Told him that I cannot open the register anymore which he understood too.
I work retail for 3 years and I am still surprised why people think if the door is unlocked explicitly means the store is opened, if even two signs are there to aid them. My mistake was that I should have locked them, and I'm glad they understood it but it's still a giant bother.