I left the house at 5 am for a 7 am flight from an airport that in good conditions is an hour away. I was so busy over the night preparing my presentation for the meeting I was going to attend at 2 pm that day, that I did not pay attention to the surprisingly heavy snowfall outside.
In the crawling traffic I arrived at the airport 5 minutes after they closed the gate.
They were going to re-book me for a flight at 7 pm that night, I told them to forget about it and looked for alternatives. I ended up using the Skyscanner app to book the earliest alternative option, an 11 am flight leaving from another airport in the area. But I quickly realised that it'd be very tight to make it to the meeting even with that flight so I had to re-schedule for the next day. The snow provided some but not great cover as it was more of a local event and not exactly the 93 super storm either, so it was a bit embarrassing. And of course I was out of the money for the alternative flight, too.
A somewhat funny mistake in hindsight happened when I was a kid. My family was going on a big holiday to Spain, it was all a bit hectic but we ended up getting to the airport with plenty of time to spare - 27 hours in fact. My dad had gotten the date wrong.