Page 766 — Dynamic Cling
It's really just "Anything that can go wrong will go wrong." But in tabletop (and indeed in general use) it's read more as, "Don't bring up the possibility of things going wrong, because then they'll have to." If you take away the pretense of blaming the universe for your misfortune, the actual rule is "Shut up or else you'll give the DM ideas."
It's time for the final boss of Fallout is Dragons, I guess!
Session 62 — The God-Executive: Libsyn YouTube
Transcript:Rainbow Dash: Well, if we can't get around it, let's just pop the question already.
Fluttershy: Maybe not just yet. It'd be bad timing after an experience like that…
Twilight Sparkle: Let's figure out how to get them home, then. That should alleviate their worries and make them more amenable.
DM: As soon as the subject is brought up, something strange happens. The Breezies, all of them except the "leader," jump to Fluttershy and cling to her mane, chattering in their language.
Fluttershy: They're probably still high-strung, huh? Actually, this works out, doesn't it? I'll let them rest under my care. Then, when the moment's right, I'll ask for the pollen.
Pinkie Pie: "It's the perfect plan! Nothing could go wrong!"
Applejack: …Did you just invoke Murphy's Law on purpose? Intentionally?
Pinkie Pie: Yep! Now both the positive AND negative outcomes would be ironic! It's anybody's game! I HAVE DEFEATED MURPHY!
Applejack: That's not how tropes work, Pinkie. That's not even what Murphy's Law is.