slipping film frames effect

Hello,

In the following film on vimeo, here is a typical old fashioned celluloid film sliding or slipping effect; as if the film has missed some of the sprocket wheels in an old fashioned film projector.

I would very much like to know how this effect is named and done.

It can ben seen in this film at 0-1 seconds and on 20 seconds and more often in this film.

https://vimeo.com/18886355

I very much hope you can help since I am very fond of it and like to apply to a film in the making.

Thanks,

Paul