First day of highschool, slipped and fell in a puddle of vomit, and some kids laughed. Sheepishly went to the bathroom and rinsed my uniform, and put it in my bag. Spent the rest of the day in gym clothes which I luckily had that day, still smelling like vomit though.
Good thing when you don't know anyone yet is nobody remembers.
My wife and I hosted a Christmas gathering at our place with a few friends as well as some acquaintances.
We had organized Secret Santa and I was given someone but on the day of the event I thought that we had been tasked with giving gifts as couples — I thought my wife had chosen a gift but it turned out that I had just totally forgotten to buy one for the person I drew.
I wrote a poem for a girl once, back in high school. It was an angsty, acrostic rant about love and relationships and shit, where the first letter of every sentence spelled out "do you want to go to prom anyway".
I walked away while she read it, trying and failing to stay cool, but I was still within her eyesight when she finished and said no.