GKG, I wouldn't worry about it too much -in fact the opposite. Here's why.
Interviewers asking things after the interview is 'over' is a common ploy to see the 'real you'. If you didn't get thrown or stumble, they'd have to think you were a robot!! So possibly the fact you didn't come out with a well-rehearsed answer working in your favour!!
SOmetimes we don't know it all, and think it's a failure not to have all the answers. But I think they just wanted to see how you deal with the unexpected, also it's possible they were trying to weed out the 'fakes' with that tricky last-minute question!!!
It's like they say on all the posts about pilot interviews... even if the interview is 'over' and it's a 'friendly chat' - treat it like an interview!!!!
I'm sure there's nothing to worry about.
My most embarrassing interview blunder??? Hmmm... probably the time the strap of my handbag got caught on the chair as I got up and made me do an oh-so-elegant stumble/stack in front of the interviewers!! Cringe City!!!
