Many years ago I was flying back and forth from Honolulu to Austin as my dad was terminally ill with about a year to live. (Wait, wait, it gets even funnier.) So I would basically leave Honolulu on Thursday night and fly back Saturday or Sunday, just to try and sort out as many things as I could in person. As you can imagine, it was totally exhausting physically and emotionally. So I get to the Austin airport for my return flight one weekend, and I'm checking into the flight at the kiosk, and it's not working. I go to the counter and I really had no patience or threshold anymore, given everything that was going on with my dad and my sleep, so I kind of unloaded on the agent. "Your machine is broken and I just need to check into the flight and get the hell home, and it would be a lot easier if you guys knew how to run an airline, but of course your machine won't let me check in" or something super rude along those lines. He asked for my info, punched it into the computer, and politely replied, "The reason you can't check into your flight is because it left yesterday."
Total humiliation. I immediately snapped back to reality, apologized profusely, explained the situation, and the agent was super nice and got me on that day's flight after all. I learned two things through that experience -- never assume you know why someone is being a total asshole, and even though it's completely exhausting, do whatever you can to spend time with your friends and family. But the embarrassment of that situation still haunts me -- I really made an ass of myself.