I honestly don’t have an idea what you are talking about, this rings no bells whatsoever.
People send notes like this once in a while, and I appreciate that they feel they should apologize for some perceived slight they may feel they have made at a convention. But I have to say, I don’t usually recall such things. Conventions are big and noisy, it’s confusing, it’s exhausting…it’s easy to say or do something that’s not entirely polite without intending to, I’ve done it MANY times myself, it’s kind of my nightmare, that I will accidentally be rude to a reader.
But I don’t recall this incident at all, so please don’t give it a second thought. If you want to elaborate, that’s fine, but the odds are, my response will be the same: thank you for the thought, and don’t worry about it even a little bit.