To accidentally become really good friends with someone before you realize that you want to be more than that, so you become stuck in the friendzone. It is a very hard and awkward place to be in, so avoid it at all costs.
"I want a girlfriend!"
"Dude, you should date Tania."
"Are you kidding me? We got totally stuck in the friendzone like, last year. To be friendzoned sucks balls."