And when they do, it's technically called a "safety", even though it's nothing like a normal safety. Here's a nice summary from Wikipedia:
In U.S. college and Canadian football, a team scores two points if it gets control of the ball on a conversion attempt by the other team and returns it all the way into the other team's end zone. For example, say Team A scores a touchdown to go ahead 6-0. A player on Team B then blocks Team A's extra point attempt, picks up the ball and runs into Team A's end zone. The score is now 6-2, and Team A kicks off to Team B from its own 35-yard line (as it would have if there had not been a safety).
College football's rules allow either team to score a one-point safety after a touchdown. Say that Team B blocks Team A's extra-point attempt, and a player on Team B picks up the ball on the 1-yard line. Looking for an opening, the player with the ball runs backwards two yards, where he is tackled. Team A receives one point, and the score is now 7-0. Team A then kicks off from its own 35-yard line.
Another scenario would be if Team B had blocked the kick and began to run it back for two points, but at the last moment a persuer from Team A knocked the ball loose. If he were to pick up the ball, run into the endzone and be tackled, Team B would score one point, and the score would then be 6-1. Not only has this never happened, but it probably never will given the rarity of a block, a last second fumble, and a bad choice on the part of whoever recovered the fumble. It is notable, however, for being the only way to score exactly one point in a game of football.
The NFL also has a one-point safety rule on conversion attempts. However, a one-point safety would be almost impossible in the NFL, since the ball becomes dead immediately if the defense gains possession of it during a conversion attempt. In order for the offense to score a one-point safety, a member of the defensive team would have to intentionally bat a fumble into the endzone without taking possession of the ball.

A great trivia question and maybe one to win a bet.
Posted by: Woody | 09/27/2006 at 08:10 AM
Indeed there is such a rule, and Northern Illinois benefitted by it in their win at Alabama a few years ago!
Posted by: Stephen Karlson | 09/29/2006 at 02:19 PM