I'm afraid I'd go along with BOO & get it over & done with.
I would simply phone the person who sent the email & say:
"Hi, so-in-so - I'm a bit busy at work at the moment, but just calling in response to the email you sent me today. I really can't think of what I must have said to upset anyone on Saturday evening, so p'raps you could enlighten me....."
If it's something you feel you should apologise for, then I'd do so. If not, I'd just laugh it off & say, "Oh, is that it?". "To be honest, I don't even remember saying that & I'm sorry that you or whoever took it the wrong way - I would only have meant it in a light hearted way".
I'm sure the person would then feel a little silly at emailing you about it & you'd probably both be pleased it was out in the open & hopefully soon forgotten.
(Although, unfortunately, there are some people out there who aren't so understanding or indeed forgiving.....but they just ain't worth it)!
Good luck....