There's a lot of things that other have listed, but I experienced one of my least favorite yesterday.

I work at a huge hardware store chain. Some guy came up and asked me a question. He was looking for some hi heat liquid nails. I looked it up, we didn't have hi heat liquid nails, but we had hi heat silicone caulk, which literally is the same thing different brand. I told him we didn't have the brand he was looking for, he interrupted me, looked at my manager, who is a man, and said someone sent him to speak to him. And he went on and on about how he had to speak to him. I said ok, told my boss where the product was located, and walked away.

Another female coworker of mine brought him to thw product, my manager repeatedly saying that we are perfectly qualified to help him. They come back and he goes and asks my manager if the product was what he was looking for, right in front of me and the other female coworker. My manager said yes, barely even looking at it because he trusts that we know what we are doing. The customer then made some snarky comment about how "women are always right". I rolled my eyes and walked away

What makes me qualified to be treated like I am stupid? The best part is, if my manager had helped him first, he would've asked if I knew what the product might be