Back in the day, there was an unwritten rule when it came to split-screen competitive games. You weren’t allowed to actively look at your opponent's screen under any circumstances. Nevermind that this was quite simply impossible for nearly everyone: in the '90s, TV screens weren't what they are today. Looking at a tiny cathode ray tube television that was further shrunk down by being digitally cut into quarters meant that you couldn't help but see everyone else's screen at once.
The rule still applied: no peeking.
Most game developers acknowledged this rule in a hand-waving sort of way that denied the reality of technology at the time. Some even joked about it. But Nintendo? Nintendo just stepped back, said "this is impossible," and just put it all right there in the open. Screen watching is inevitable, so why bother trying to hide it?
In the Super Mario Kart instruction booklet, Nintendo lays it all out in black and white. In order to win at Battle Mode, Nintendo recommended to "always keep an eye on your opponent's screen." That's a direct violation of the unspoken rule, and it caused more than its fair share of conflict.
More than one friendship was lost on the crucible of Super Mario Kart. Not much has changed over the years, but at least we know the reason for all of that childhood angst.
We have Reddit user flabby-doo-dad to thank for this relic. The uncovering of this information is causing quite the controversy on the gaming subreddit. The top comment says: "Welcome to competitive gaming. If it's on the screen, it's gettin' seen." However, others replied with homebrew alterations to the split-screen formula: "My friends and I made a cardboard divider to try to prevent watching other screens."
Some argue that the original Super Mario Kart was designed explicitly with screen watching in mind. Considering the manual says it, those people are probably right. Still, that doesn't mean those same screen watchers aren't dirty cheaters all the same.