I was performing a trick with a normal deck, I had them select a card, and I controled it to the bottom using a overhand shuffle. Then the spec suddenly yelled "hey, my card's right there on the bottom!" What should I say or do in this situation?

"It is? Maybe you shouldn't have told me, now I know where it is." Then I would do some fancy false cuts (fancy for me anyway ) some riffle shuffles and hope for the best. But you definately need to learn some other controls and figure out what they spotted.

That's exactly what I would've done. Act like you didn't know...

"It is? Thanks, this tricks going to be really easy to do now"

"Good thing you told me... I thought it was the five of clubs."

"O... well that wasn't very magical, let's try another one."

"Is it really? I'm going to have to use this shuffle more often."

"No it's not." SHOW CARD "Yes it is!" "Can't you just go along with my lie?"

"OK, let me put it on the table then." Do that move where u place the second card from the bottom on the table... not the regular glide, but that fancy one in the Ninja 1 preview... follow with hot shot cut... "Told you it was in the middle."

"I'm not done shuffling." control it to the top instead and do a different effect

WOW, wasn't that cool!