In yesterday's Red Sox - Blue Jays game, Boston's Gabe Kapler was on first base when teammate Tony Graffanino hit a long fly ball. Kapler started running hard, and as he rounded second base, he fell to the ground: he had ruptured his left Achilles' tendon. Meanwhile, Graffanino's hit went over the fence for a home run.
Sports rules always have a hard time dealing with injuries during a play. Baseball has a special provision in this case:
Rule 5.10 (c) (1) If an accident to a runner is such as to prevent him from proceeding to a base to which he is entitled, as on a home run hit out of the playing field, or an award of one or more bases, a substitute runner shall be permitted to complete the play.
So, the game stopped as trainers helped the injured Kapler off the field, while Graffanino waited on first base. Then Boston was allowed to send in pinch runner Alejandro Machado, who started at second base, then ran to third and home, while Graffanino followed him to complete the home run.
Of course, if the ball hadn't gone over the fence for a homer, poor Kapler would likely have been stuck on the ground helpless while the Blue Jays got the ball and tagged him out.
(Thanks to Marshall Clow for telling me about this play.)