Just found this thread! I no longer do, but I used to manage a LFS, and for the time I was there boy did I hear a few good ones.
My very first day...a lady was walking up and down the aisles, looking at the saltwater fish. I asked her if she needed any help, and she said no, she didn't have a tank but was just admiring while her kid was next door finishing his music lesson. She pauses at one of the tanks that had a few small Niger Triggers and goes wide-eyed. She hurriedly walks up to me and points at them and in a whisper says "What a pretty fish, but that's a really horrible name for them!"
It took me seriously five whole minutes to figure out what on Earth she was talking about. Then the lightbulb went on and my jaw hit the floor!
I also grew to hate the Biocubes in my time at the store. People began to equate them with "miracle tanks" that because it was this all inclusive system, it meant water would magically cycle and any and all fish, inverts, etc could live in them. Apparently, people think they function a lot like the Tardis from Dr. Who, and they have no problems stuffing Tangs, triggers and other completely inappropriate fish in them! I had one customer come into the store and ask me to bag up a real pretty little Desjardini Sailfin we had. I asked him how large his tank was, and he points to the 29ga Biocube we had set up on display and says "That Big" I pause and tell him that although the fish is little now, it will rapidly outgrow a tank of that size, but I'd be more than happy to show him some smaller, more nano-appropriate fish.
Well he gets miffed and tells me all his OTHER tangs are doing quite well in there! My stomach sinks as I ask "Other...tangs?" He then goes on to tell me he has a yellow tang, purple tang, one dori fish, a nemo fish and a damsel in his Biocube. I flat our refuse the sale of anything. He gets angry and says he wants another fish! Long story short I end up telling to take a left out of the parking lot, go a few miles to the other pet store down the street because he wasn't getting them from me!
Not fish store related, but I will never forget this one. Years ago for a summer job I worked on a lobster tour boat in Maine. We'd take turns giving the tours and working in the ticket booth. It was my turn in the booth and I was chatting with a nice lady that was waiting for the boat to come in. She tells me she had been there earlier in the summer, and this was her second time back.
Then she asks me if we had had a lot of rain that summer. I pause, puzzled and say "No more than usual, why?"
She points to the cove behind me and says "Well when I was here earlier in the summer, that cove was almost empty. Now it's completely full of water!"
I just smile and nod and said "Yeah, I guess we had a lot more rain than I thought!"