Anyone who follows sports is familiar with the concept of a borderline play—the move or technique that straddles the line between playing hard and cheating.  The extra step toward the basket, the intentionally-unintentional pick; the inside fastball designed to scare, but not strike, the batter.  We expect these plays because we understand that sports are competitive, and that competitors will push the boundaries on what they’re permitted to do in order to win.

Sometimes, however, standards change.  What was once an accepted, if borderline, tactic becomes recognized as unhealthy or unsafe.  Major League Baseball didn’t formally ban the spitball until after Ray Chapman was killed by a tobacco-covered ball, which was allegedly hard to see in the game’s lighting.  The NHL permitted direct hits to players’ heads—infamously including Scott Stevens’s brutal, but completely legal, hit on Eric Lindros in the 2000 Eastern Conference Finals that concussed Lindros and changed the course of his career—until research made the risk of CTE undeniable.  The NFL has made a host of changes, from banning horse-collar tackles to changing the rules for tackling quarterbacks—at least in part due to a season-ending injury superstar Tom Brady suffered due to a legal tackle by Bernard Pollard—to introducing the concept of “defenseless players” in the name of player safety.