Here's an interesting fact: It's possible for twins to have different fathers.

Let that sink in for a minute.

While rare, it is possible for a woman to get pregnant while she's already pregnant. According to this article on babycenter.com:

In an odd phenomenon known as superfetation, a pregnant woman releases an egg a few weeks into her pregnancy. The second egg is fertilized, and the woman is then pregnant with two babies simultaneously. (Don't confuse this with superfecundation – when two eggs released during the same cycle are fertilized at separate times.)

How bad would that suck if your wife gave birth to two different colored babies? Or, if you have a one night stand with a chick and don't use a condom because she assured you she's a month or so along?

Of course, when the paternity test comes back, you can always hope that they tested the baby that's not yours, but that's a gamble that you should pass on.

Basically, don't have sex with a pregnant woman that's not yours, and always wrap your willie when you are not trying to have kids of your own. Always.