So as most TIFU posts, this didn't happen today but last year. I regularly go to a local grocery store in town to get the essentials like everyone else. This is one of those stores that after a certain time in the evening only the self checkout isles are open with one or two employees watching over the customers, helping with coupons, and checking for IDs for alcohol. Well as a regular customer I noticed that most nights a mentally disabled man( let's call him Dale.) was running that section almost everytime I stopped by. He was always courteous to me and we would talk about different craft beers the store would cycle through. (Yea, the guy loved his beer. Go figure.) Now I have a soft spot for people like this especially when they're doing their best to make a living for themselves and doing the best with the hand life has dealt them. Soooo..one night I walk in to get some beer for the night since I didn't have work in the morning. I make my selection and as I'm checking out at the self service isle I hear a commotion taking place a few feet ahead of me. It's dale being berated by a much younger male employee. I overheard dale getting chewed out for being late. "YOU WERE SUPPOSE TO BE HERE AT 10:00!!! WHAT WERE YOU THINKING?! ARE YOU STUPID?!? I HAVE TO GET HOME TO MY KID!!" etc etc. No one deserved to take this abuse dale was recieving. I hesitated for second, and looked at my watch. It was only a few minutes after 10:00. So I realized I needed to stick up for dale. ESPECIALLY since this other coworker was going ballistic over just a few minutes. I couldn't believe he was that angry over 5 minutes. I began cursing the other employee out for the way he was talking to him and tried explaining to him it was only a few minutes past 10:00 and he shouldn't be making such a big deal about it. I really wanted to make this guy feel stupid for being such an asshole to this poor man for being just a few minutes late. I could feel the warmth of other customers eyes staring at me as I let this guy have it, but I didn't care. After my 30-40 second tirade on this guy for making a big deal over five minutes he calmly said "Actually, it is 11:05. He's a hour late. Did you forget about daylight saving time??" My throat immediately swelled and I could feel my face rush to red. I had completely forgot about it. I didn't even say a word and walked right out. Needless to say, I haven't been back to that store. I felt like the biggest dick ever and was completely embarrassed as I turned out to be the bigger asshole, even if I was just intending to stick up for dale. Sorry dale, but we were both stupid that day.

Tl;dr. Tried sticking up for a mentally disabled man by berating his co worker for berating him for being a few minutes late to his work place. In actuality, myself and the mentally challenged man forgot about daylight saving time. Fml.

EDIT: I assume dale forgot about daylight saving time as well. I never got a chance to ask him in my rush to flee the scene.