I had one in ORD, UA. I was fairly new to JS and knew that you have to be courteous however that day I just tired, wasn't feeling it. Showed up at the gate about 1 hour before departure time. Picture a chain smoking, alcoholic Denny's waitress, I show her my I.D and Passport with minimal conversation. She mispronounced my Eastern European last name, I corrected her in a non confrontational but somewhat firm tone. Maybe in her less then high school educated 50 year old mind she was thinking this 20 something son of a B$tch. Then, here we go, she claimed she couldn't find my airline in Cass the she said I was being denied. I told here I just jumpseated on UA 3 days ago, she said she didn't care she only believed the computer. I had to call the 800 number, the guy in the background said I was fine and to let him talk to the agent, she said she wouldn't talk to the guy because she didn't like the tone that I described the situation making her look incompetent, which I never did, I just said that she wasn't accepting me. So by this time the guy in the phone got angry and just hung up. Thankfully the CA showed up, I introduced myself, he went by the agent she explained what was going on. I told her try again one more time, reluctantly with the CA standing next to her she tried it again, what do you know the computer "magically" accepted me. Unbelievable. Anyways at this point I knew she was furious, I was getting on but she made sure I was the very last person on. Later found out that she was one of those high and mighty types, and overheard her talking to another customer about alcohol how she used to work at a bar. Very unprofessional person, even though it's been a long time I could probably still i.d her. What made me even more angry is that after the FAs showed up she went back behind the counter to tell them about me. When I went in to check with the CA he told me that the lead FA wasn't impressed with what I did. Inside I was boiling because I hadn't done anything, she was the one that was playing the cruel joke. It's hard to imagine that gate agents can play these games, especially with tired overworked pilots who are trying to go home.