Okay, so the 136-year ban is actually a glitch and not a purposeful sentence, but it stinks all the same, especially since there is (as of right now at least) apparently no way to fix it. The glitch can be avoided however, since it seems to be happening only to players who quit online matches early or excessively target an individual in the game's four-player For Fun mode. So, don't be a bully, don't rage quit, or if you're really worried about this happening to you, just don't go online at all, basically.
Reddit user "rstevoa" theorizes the glitch is caused by the game's code counting ban time too low, which would in turn cause the code to loop around to the highest number of seconds it can assign to a ban. Of course, that's just a theory - who knows what dark sorcery actually goes on at Nintendo?