I was doing the biddle trick, and I did the most bone-headed thing possible... I forgot what their freakin' card was. I was so pleased with the convincing controlled shuffle I did, that when I did the biddle move their card flew right by me, I spaced out, and forgot what it was. A classic case of biddling under the influence.

No sweat. Some quick thinking-on-my-feet saved the trick. I hold the cards in my card, and make two wrong guesses on purpose. I act disappointed and say, "Well, what was your card?" They tell me "Seven of clubs." I hold out a card showing it's back and say, "Oh, the seven of clubs here, that was my next guess! Well I can't do that trick very well, but here's a trick I can do. Take your seven of clubs (show back of indifferent card) and look..." (snap--it vanishes) "Look in the deck you're holding, right in the middle, there it is, face up."

In the end the effect was really not much different from the normal Biddle trick, and nobody guessed for a second that anything had gone wrong at all. A good example of where you should not panic just because things didn't go exactly by the script.

You do it a little different from the way I do it. Just to make sure we're talking about the same trick, the effect is that you hold a packet of five cards, one of which is her chosen one. Then it disappears from your packet and appears face up in the deck which the spectator's been holding all along.
Basically, the way I do it, I first affirm that the person saw her card in the packet of five. Then, after a little move, I say I made her card disappear and show the remaining four. Give that a moment to sink in, then say that I made the card magically jump to the deck face up and have her spread the cards in her hands.
You could do something simular. If you suddenly realize that you don't remember the card, just show the remaining four and then tell her to spread through the deck.

Yes, we're talking about the same trick, you just handle it a little differently. I like to present it with the old plot of "magician screws up trick, then redeems himself", especially as an opening trick where the spectator expectation is at the lowest possible point. I do that by intentionally making two wrong guesses before I get it right, and when I get it right I hold up an indifferent card, back to audience, and miscall it. They think that the (lame) trick is over, but then the real trick begins.