I had a domain back in the mid 90's that was a three-letter WORD with a .net extension. Due to unforseen circumstances and really no value to domains at the time, I let it go/lost it.

What domain did you drop or lose that you later on regretted?

__________________
What is in a NAME?

What's the three-letter word domain that you let get away?

What is it now?

I cannot say that for myself since I have been into this business not so very long. Unforseen circumstances can make our past mistakes regettable in the future. It always comes at the end. We could have prevented it had we foreseen what would happened beforehand.