What's your favorite obscure and/or archaic sports rule?

Barbados vs. Grenada in the 1994 Caribbean Cup

The match started off routinely, and Barbados scored two goals, establishing the two-goal winning margin they needed. In the 83rd minute, however, the game changed when Grenada scored a goal. This late goal would take Grenada through to the finals unless Barbados could score again. Barbados attempted to score for the next few minutes, but because they were unable to, they switched to a different strategy, of tying up the game so that they could try to achieve the two-goal margin in extra-time. In the 87th minute they stopped attacking, and the defender Terry Sealey and the goalkeeper Horace Stoute passed the ball between each other before Sealey intentionally scored an own goal.

Now the game was at 2–2, with just three minutes of normal time left. The Grenadian players caught on to Barbados's plan, and realized that they would advance in the tournament by scoring a goal in either net. This left the match in the highly unusual position of no clear side of the field for one team versus the other, but rather one team trying to score a goal in either net, and the other team trying to defend both. For the next three minutes, Barbadian players successfully defended both sides. Still drawing at 2–2, the game went on to extra-time, where the winning "Golden Goal" would count double — so Barbados only had to score once to qualify for the 1994 Caribbean Cup. Thorne scored the winner for Barbados and they advanced to the next round. The score was 4-2 yet they had only scored 3.

I don't have one off the top of my head, but if you like this topic you should check out SB Nation's "Weird Rules" series on youtube. Actually, most of SB Nation's stuff is good. Beef History, Collapse, Rewinder, Chart Party, and The Worst are all good or great.

In 1976, Muhammad Ali fought Japanese pro wrestler Antonio Inoki in a precursor to MMA. There’s a lot of confusion and conflicting statements about the actual rules for the fight, and whether or not it was originally intended to be an exhibition match or a real unrehearsed fight. What happened became a huge embarrassment. Due to the rules of the fight, Inoki spent most of the match on his back or crawling around on the floor, kicking at Ali’s legs (he was not allowed to kick from a standing position). He was also not allowed to execute a proper wrestling takedown to force the fight to the ground. Meanwhile, Ali was unable or not allowed to punch his grounded opponent, and grew increasingly frustrated while his leg swelled up from repeated kicks. Ali scored some hits standing, but Inoki would just get back on the ground again. In the end, the fight was scored a draw, and fans of either fighter got neither a win nor an entertaining fight. It was bizarre.

In general, early mixed/hybrid martial arts contests had to do a lot of experimenting to come up with a ruleset that made sense. I guess it was the same for most sports, it’s just that MMA is young enough that the pioneer days are still within living memory.

My absolute favorite obscure or archaic rule, though, comes from chess. I guess you could argue whether chess is a sport (the IOC says yes, many sporting bodies say no). I would call it a mind sport, but I’m not here to start that debate. I think it’s within the spirit of the OP’s question.

Anyway, in the 1970s, a couple of people independently discovered a loophole in the rules of chess as written. Specifically, vertical castling. Under very obscure circumstances, namely if the king is still on its starting square and the e pawn underpromotes to a rook, you could arguably interpret the rules such that they allow for vertical castling. There is no recorded game where anyone attempted this creative loophole, but chess composer Tim Krabbe published a puzzle in which this is required for the solution. The world chess federation subsequently amended the rules to close the loophole. The circumstances where this would have been possible are extremely unlikely to occur in any real game, but it would have been funny to see someone attempt it.

If you hit a game-winning home run with runners on base, all of the runs count, unless the batter doesn't make it all the way around the bases. Two famous examples:

Harvey Haddix's 12-inning perfect game:

A fielding error by third baseman Don Hoak ended the perfect game in the bottom of the 13th, with the leadoff batter for Milwaukee, Félix Mantilla, reaching first base. Mantilla later advanced to second on a sacrifice bunt by Eddie Mathews, which was followed by an intentional walk to Hank Aaron. Joe Adcock then hit an apparent home run, ending the no-hitter and the game. However, in the confusion, Aaron left the basepaths and was passed by Adcock for the second out and the Braves won 2-0. Eventually the hit was changed from a home run to a double by a ruling from National League president Warren Giles; only Mantilla's run counted, for a score of 1-0, but the Pirates and Haddix still lost.

The Grand Slam Single:

The next batter was Mets third baseman Robin Ventura. Ventura crushed the 2–1 pitch over the wall in right-center for an ostensible grand slam, winning the game for the Mets and driving the Mets players and fans into a frenzied celebration. Ventura, however, never reached second base as Todd Pratt, the runner who was on first, picked up Ventura in celebration. Subsequently, Ventura was mobbed by his teammates, never finishing his trot around the bases. Because he failed to touch all four bases, the hit was officially scored a single. Roger Cedeño, the runner on third at the time, was ruled the only runner to have crossed home plate before the on-field celebration began and the Mets were awarded a 4–3 victory. Thus, Ventura was only credited with a single and one RBI.

The Sean Avery rule in hockey.

https://youtu.be/_0gQMVM8iAo

'Gentlemenne should divest þemselves of þeir Rapiers ande Hand-cannones ere entering ye noble Pastime of Whist'...