Most photographers can recall a number of such instances.
There are ways in which the film can be retrieved from a cassette even if it is of the unopenable kind such as most Kodak and now Ilford types. You might also want to take a look at the PhotoForum's FAQ files. In section 28 there are several methods, in addition to these, described. Among the improvised methods a one uses two-sided sticky tape which is applied to a piece of film and inserted through the felt trap into the cassette with the idea of sticking it onto the film inside and allowing you to pull the film back out of the cassette. A second way, which I learned from Larry White of Modern Photography, is to stick a piece of, preferably, undeveloped film onto your tongue and moisten the emulsion side so that it becomes as sticky as if you had used double sided tape and then push this film, wet emulsion down, into the casette hoping that, first, the swollen emulsion will adhere to the film inside and that, second, yanking or slowly pulling on the remaining film will withdraw with it the film from within.