I was only 20 when I got married, 21 when I started having DC. I didn't realise that when guests come, because you've recently got engaged/promotion/new baby etc and they bring champagne.. you are supposed to open it while they are there. I just saw it as a gift like any other, it didn't occur to me to open it them. I think I blundered in this way on numerous occasions and still feel embarrassed when I think of it. I don't drink really anyway so it's not like I was keeping it back out of greed.
In the haze of several DC back-to-back I also had friends come to visit, sometimes from up to an hour away and a couple of times I offered them nothing more than tea/coffee as I think I just presumed they were calling in for an hour. But then it ended being for longer and I realised after they'd gone that it was after lunch and they'd surely gone away hungry.
Once I had a friend drive two hours with her DC to visit and I'd burnt the quiches I'd made so just tried to bluff the rest of lunch but it was so sparse and the house must've smelt of burnt food and I still feel mortified and think why did I not just say what had happened and also surely there was more food in the cupboards that I could have rustled up.
I'm sure there are so many others. I feel I went out into the world quite unprepared on what the 'done thing' is but also I think perhaps having three babies in 4 years sort of unhinged me temporarily in the sense I didn't really think about things like social niceties as well as I should have? Sometimes I see threads on here about CFs and I think 'oh god was I CF back then?' I never wrote a single thank you card for wedding, newborn, christening gifts etc.
There were also many occasions I did all the right things, made guests lunches, dinner etc - but I just cringe at the ones where I seem to had temporarily forgotten how to behave!
Do you have things from the past that you still feel embarrassed about? I have other memories too of falling over during a work presentation and other moments where I wanted to sink through the floor but those occasions are no reflection on my character so it's a different kind of embarrassment.