On Friday, Cubs right-hander Jake Arrieta tossed six scoreless frames against the Brewers (CHC 6, MIL 1) in his final outing before the NL wild-card game against the Pirates. In doing so he lowered his 2015 ERA to 1.77 and his second-half ERA to an astounding 0.75. On that front ...
Since the break, he's struck out 113 batters in 107 1/3 innings while issuing just 22 unintentional walks and allowing two homers. Over that same span, opposing hitters have managed a paltry line of .148/.204/.205. Yes, that's a .409 OPS allowed, and, yes, such a mark strains belief.
As for his second-half ERA, let it be noted that lefty Ferdie Schupp of the 1916 Giants pitched to a second-half ERA of 0.71 in a qualifying number of innings. However, just 10 of his 22 second-half appearances were starts. As well, there was no All-Star break in those days, so the dividing line isn't quite as tidy.
So call Arrieta's second-half mark a modern record if you like, but it's been an astounding run in any context.