When coitus ends before the male ejaculates. An extremely unreliable method of birth control, as sperm can leave the penis before ejaculation. Commonly seen in porn movies as a way to supposedly prove that the man is not faking an orgasm by showing a money shot.

I thought this was when the phone rings or someone knocks at your door during

hot hot sex.

Though sometimes the sex isn't so hot, and the surprize visit can be a blessing.

In Italian we call this "salto dell'oca", which translates nicely to "goose jump". Not that I know much about the sexual habits of geese - usually birds are not that exciting that way.

coitus interruptus is a fecundity reduction method. Its failure rate is very high, and it probably helps control the population-wide birth rate somehow, even if it is no guarantee for the individual couple.
Actually, I guess that a fertile couple practising C.I. is almost guaranteed to produce offspring sooner or later.

Coupled with "natural" methods (that tell you that you can have sex only on certain days) it makes for a really miserable sex life.
Unfortunately, in some places social pressure and economy combine to make this the only contraception method.