Page 869 — Racket Rocket
As has been pointed out before, you don't necessarily need to consider Natural 20s an automatic success, especially if it's something completely impossible.
However… they are fun when they come up, and I'm of the opinion that when they do, even if it's annoying… you gotta throw them some kind of bone. Even if it's ultimately a failure because they were doing something impossible, you should at least make it a little bit special regardless.
Transcript:DM: So… what do you all actually want to do before the Competition starts?
Pinkie Pie: I say we follow the money!
DM: Huh?
Pinkie Pie: Well if they're trying to fix the Competition, they're probably looking to win big on the gambling circuit!
Rarity: That… WOULD make sense… if–
DM: People might bet on Wonderbolts races, but not on the Best YOUNG Fliers Competition. Also, it's illegal.
Pinkie Pie: Well, duh! Of course it wouldn't be legal gambling!
Twilight Sparkle: Sure, we'll just start "asking around" about illegal gambling.
Pinkie Pie: That's what Streetwise is for!
DM: You're not in Ponyville! Your cutie mark bonus doesn't apply!
Pinkie Pie: It's still good!
DM: Fine! Whatever! Search for this mythical illegal Best Young Fliers gambling ring.
Pinkie Pie: <roll> <gasp!> Natural 20!
DM: Ffffffffffff–
Fluttershy: Isn't that a "spectacular success," too?
Applejack: Guess we're off to the races!
DM: FFFFFFFFFFFF–