I'm sure many of you have had a similar experience, so I'm curious as to how you handled it.
I give blood on a regular basis but up until a few months ago, all my ID was still under my female name.  I finally got it changed, received a new card, and had the sex changed to male in my records at the clinic.  Every time I've gone since then has been much more comfortable.  I'm always called by my chosen name.  I dress like a guy and am pretty small chested to begin with so I just look like a younger guy. 
However, I went a couple days ago and everything was fine until I was sitting down waiting for my name to be called.  Out of nowhere, I heard my old name and I responded out of pure habit.  It was a guy that knew me two years ago when I was at college because he saw me play baseball on the women's team.  To top it off, I was sitting next to this guy's mother, who was also waiting.  Once I had reacted, it was too late and I wasn't sure what to do.  So I had to sit there for the next ten minutes having a conversation with this guy that I didn't even remember but who obviously remembered me! 
So I guess my question is, when this happens is it better to pretend that we have no idea who they're talking about? Or is it best just to grin and bear it, and accept the fact that even after I'm on hormones and have had surgery, this could still happen because there's always the chance someone will recognize us.
I apologize for the long-winded post.  I was just feeling really embarrassed about it and I was hoping for either some words of advice so for some shared stories so I know that it happens to everyone and it's not the end of the world 

In those circumstances, ignoring the people might cause them to come over. The guy was obviously pretty certain it was you.

Can you come out and explain to them what has happened?

what is wrong with telling them you are 'having a sex change'?  They will immediately understand sexchange even if the fine details is beyond them.

I just ignore the person or act like I don't know them. Sadly, I usually don't remember them. Once your name and gender is changed on IDs and documents it's hard for them to prove otherwise.

Id just tell them. I mean, if it isn't somebody ur buds with, what can they say to u other then an awkward "ok"...

Did any of this involve your gender?  By that I mean, he called you by your old name in a public waiting room.  If no one else there knew your male name or your trans status was he outing you?  Was he even aware of your trans status (ok it sucks that it means you didn't exactly pass as you wanted to of course)?

Sometimes you just have to go along with it.  It sucks that obviously he didn't read you as male, but then its your choice to out yourself or not I guess.