"The objective of Stampede is to round up all of the cattle you encounter. To do so, the player must lasso each one in order to capture it. The player is initially only allowed to let two cattle pass; if a third one slips by, the game is over.
An important caveat to this is that the player gets one extra free pass for every 1,000 points scored. So, if a player were at 2,000 points and had not yet let any cattle pass, then he or she would be allowed to let four cattle pass before the fifth one ended the game (assuming she did not reach 3,000)."
Source: Wikipedia, "Stampede (video game)," available under the CC-BY-SA License.