I've received that Error 99 message several times on many different occasions. And you're right, it's always at the WRONG time!
One time, my wife and I went to a concert that features about a dozen different singers. My wife was really looking forward to 1 particular singer who was toward the end. In the meantime, I was taking pictures all throughout the concert with my Canon 40D. Of course, minutes before my wife's favorite singer comes out, my camera decides to give me an Error 99!!!

To make matters worse, after the guy did his couple of songs and left the stage, my camera goes back to its normal, regular self, and no more error messages for the rest of the night!!!
Yes, in general, when you get the Error 99 message, you should turn off the camera, remove the lens, remove the battery, put the battery back in, put the lens back on, and turn the camera back on. For some reason, this works for me whenever I get that error message. Yes, there's also that thing with the pencil eraser gently rubbing the metallic lens contacts, but I haven't needed to do that...yet.
My other Murphy's Law vacation story. We were at the Magic Kingdom in line to take pictures with the princesses. I was next in line to see Cinderella, and I was as giddy as a little school boy. My wife helps me take my picture with Cinderella. But sure enough, as I'm posing arm in arm with Cinderella, my camera's shutter BROKE!

AAARRRRRGGGHHH!!!! How did my 40D know that I was standing right next to Cinderella for the 2nd most important photo of my life

?!!??! (the first being my wedding photo with my wife) My wife "claims" she didn't do it, although I could see the sparks of jealousy in her eyes.
Turns out, the shutter's secondary mirror broke, so I had to send it back to Canon, and $200 later it was all fixed. Luckily, I had a back-up dSLR in the hotel room and went back later on during the vacation to get my picture with Cinderella.
Yes, these things can and DO happen. Always at the WRONG time. How does it know?
(yay! my 350th post)