There are several contenders for this but I think the top two are:
The year when I had newly learnt to drive, and went to fetch my grandma to bring her to our house for Xmas day. Despite df having invested much time in educating me on how to maintain/run a car, I ran out of petrol broke down due to circumstances beyond my control.
These were the days before mobile phones. Grandma lived about 15 miles away, and we petered to a stop at mid point. So not close enough to walk back to hers, and not near enough to walk on to ours. Plus she was in her 80's and had not long had a knee replacement. Plus it was cold. Plus she didn't want me to leave her when I suggested I go and try to find a phone box or a house who's door I could knock on.
In the end, dm and df having become very worried, df set out to look for us. He found us arguing in my broken down car, and after checking we were ok and sorting the petrol situation, gave me the worst telling off. Naturally, when we got home, Christmas dinner was "absolutely ruined" which, again naturally, was my fault so dm then gave me the worst telling off. I spent the rest of the day sulking in my room.
The second contender was about 10 years ago when our Aga broke down on 23 December and nobody could fix it until January. We decided the best solution was to try and book Christmas lunch out somewhere. I wasn't very happy about this as it was going to cost (there were 6 of us) but we had little choice.
We'd sat ourselves down at the table in the restaurant, pulled the crackers and put on our paper hats, when a woman I worked with walked past. I stood up to hug her and somehow knocked the (lit) candle on our table over. The debris from the pulled crackers immediately caught light and before we knew it there was quite the fire going (I'm cringing just remembering this). Dh and eldest ds started throwing glasses of water all over the table, but let's just say it was quite some time, and much upheaval later, before things had calmed down. I was absolutely mortified for having caused such chaos and for potentially having ruined lots of people's Christmas dinner, but most people were very nice about it ...