Water polo: no groin-grabbing.
Water polo players can be kicked out of the game for kicking, elbowing, and any other "act of brutality" — which reportedly includes grabbing an opponent's crotch. Also not allowed: splashing an opponent in the face.
Fencing: leaving the piste is admitting defeat.
In fencing, leaving the piste (the platform where the match is held) means you accept the judges' decision about the match. When Shin A Lam lost to Britta Heidemann last Monday on a questionable call, she disagreed with the judges, so she stayed on the piste — for 45 minutes. BuzzFeed's Kevin Lincoln called it "the filibuster of fencing."
Open-water swimming: no jumping on the bottom.
The official rules for open-water swimming (which includes the men's and women's 10 km marathon) allow swimmers to stand on the bottom during the race if they want to. However, "they may not walk or jump." Also prohibited: swimming too close to the escort boat to take advantage of its wake. Swimmers are allowed to use grease to prevent chafing of their armpits or inner thighs, but only if it's not "excessive."
Women's gymnastics: leotards must be "elegant" and "proper."
According to the official rules, each female gymnast must wear a "correct sportive non transparent leotard or unitard [...], which must be of elegant design." The neckline must be "proper," extending down "no further than half of the sternum and no further than the lower line of the shoulder blades." No deep v's or scoopnecks allowed! Jewelry is also prohibited, except for small stud earrings.
Rhythmic gymnastics: if your hoop gets stuck in the ceiling, you have to keep going.
The rules for rhythmic gymnastics state that if any of the apparatus used in the routine — this year the hoop, the ball, the clubs, and the ribbon — breaks or "gets caught in the small beams of the ceiling," the gymnast doesn't get to start over. She won't be penalized for the breakage or ceiling-sticking, "but will only be penalized for the consequences" — which, given that much of rhythmic gymnastics is about correctly manipulating the apparatus, could be severe. Rhythmic gymnasts can also get deductions if their ribbon ties in a knot, or if their hoop vibrates in midair.
Beach volleyball: opposing teams must wear different-colored uniforms.
The sport's official rules stipulate that, "If both teams arrive at a match dressed in jerseys of the same color, a coin toss shall be conducted to determine which team shall change." Players are supposed to be barefoot, but can wear shoes or socks at the discretion of the ref. In 2000, the rules specified a maximum size for female players' bikini bottoms — that has since been relaxed.
Equestrian: a nick in the leg can disqualify a horse.
Show-jumping horses are checked before competition for any heat or sensitivity in their legs. This is to keep riders from using substances like chili pepper to make their horses' legs especially sensitive to pain — and thus give them extra incentive to jump over rails rather than smashing into them. But small cuts that horses sustain as part of normal activity can also produce heat and sensitivity. Rider Tiffany Foster says that's what happened when her horse was disqualified over the weekend — and event officials appear to concur. It looks like a nick "like a paper cut" (in Foster's words) will stop her from competing.
Shooting: no smoking or cell phones.
The official rules of Olympic shooting specifically prohibit smoking on the shooting range or by spectators. Competitors, officials, and spectators also have to turn their cell phones off, and "notices must be displayed to indicate to spectators that mobile phones must be switched OFF and flash photography is prohibited until all competition is completed."
Race walking: one foot must touch the ground at all times.
What sets race walking apart from running is the requirement that the athlete "maintain contact with the ground" at all times. However, it's apparently mathematically impossible for race walkers to achieve the speeds they do while actually following this rule. Maybe the sport needs to think about changing its judging, which is done entirely by the naked eye.
Wrestling: hankies required.
According to the rules set out by the International Federation of Associated Wrestling Styles, each wrestler "must have a cloth handkerchief with him during the whole of the match." Wrestlers must also wear singlets that are either red or blue — a mixture of the two colors is "forbidden." Female wrestlers may not wear underwire bras.