Things happen, like right after you load a brand new roll of film only to find that the camera battery is dead and, in the process of rewinding the film, the leader goes right into the canister. That's just one example that over time has resulted in a handful of unshot, perfectly good rolls of film that can't be used because the leader strip is inside the canister. They've sat in the refrigerator waiting for me to rescue them when the time was right, like today.
I found several methods for retrieving the film leader from the canister, picked one, and it worked. Using the leader from a roll of old color film, I licked the emulsion, stuck it in the canister at the bottom (the end with the little knob, because that's where the leader is), turned the knob counter-clockwise a few times, and quickly pulled out the rescue leader. The film came out with it on the first try (I think I got lucky).
Insert the moistened rescue leader into the canister, positioned towards the end where the knob is located.
Quickly pull the rescue leader out of the canister. Hopefully, the lost leader or film will come out with it. As you can see in this example, the film had torn, causing the problem.
Here are some other ways to retrieve that little strip of film if you find yourself in a similar predicament:
1) Use a film leader retriever. With an average cost of $5, these little metal gadgets fit into the canister's film trap and grab the leader quickly and easily. Here's a demo: http://www.youtube.com/watch?v=Voxdvu7Loj8 If you don't have one handy and there's a photo lab nearby, they will usually be nice enough to pull out the leader for you. If film is your thing, maybe consider adding one to your collection.
2) Use my method above but with double-sided tape. I also read that inserting a piece of tape will work, but that sounds like it could be a sticky situation if the tape tears.
3) This method looks interesting, though it requires quite a bit of prep work and patience (which means I'll never try): http://people.rit.edu/andpph/text-retriever.html
I hope you find these tips useful. As always, if you've got tricks of your own you'd like to share, please do so in the comments!
attached photos by me