"It's quiet. TOO quiet." -bullet whizzes past his head- "And suddenly it's very loud. I liked it better when it was quiet!"
And another one from Church:
Church: As long as we're all happy, as long as we're all calm, and there's nothing that will get us excited or riled up. (KABOOM) O'Malley: Attack! Attack! Church: I could almost feel that coming... right as I finished the sentence.
"Nothing short of an aspirin overdose will stop this plan from succeeding!" -Said by Captain Butch Flowers, who apparently died of an aspirin overdose administered by a time-traveling Church.
'Shrooms'. In "Supernatural Happenings", Red boldly insults a house full of ghosts, thinking they'll leave if he stands up to them. The camera HUD even changes to say "TEMPTING FATE" as he challenges them.
In "Funk Master Z", Twilight praises Ponyville for going an entire week without driving away some outsider, and she expounds on how happy she is that the others are becoming more accepting. As soon as she stops talking, she realizes her friends are hiding from Zecora because she's different. Cue a Rapid-Fire "No!" on Twilight's part.
In "Little Miss Montage":
Twilight: I think karma kicked in and the universe is finally giving me a happy, stress-free day! Rainbow Dash: Look out below! [Crash] Twilight: Thanks, universe. Thanks a lot.
Parodied in GEOWeasel, where Nar has to say the fate-tempting line twice for it to take effect.
In a later episode:
Nar: What could possibly go wrong?
Weas: A lot, now that you said that.
Parodied and subverted in an episode of Pucca. The introduction of the ship Gigantic by the captain includes calling her "unsinkable" three times, and the floor show in the dining hall involves a song including the lyrics "Because the ship can never sink." She hits the iceberg without incident, prompting the captain to cry "I told you it was unsinkable!" Later Double Subverted.
After the "Goku vs. Superman" fight, Boomstick happily exclaims "It's over! It's finally over! We never have to hear about it again!". Cue the Internet Backdraft. The fight got a rematch later on.
"Ratchet and Clank vs. Jak and Daxter": After Ratchet and Clank crash their ship into Jak and Daxter's house, Ratchet reassures the latter duo that "this isn't so bad", since the house was still standing and mostly-intact. Almost immediately the spaceship partially stuck within makes an explosion that utterly destroys the house.
Cameraman: It's pretty much impossible to find an ugly one in this bunch!
(cue Judith Dinsmore and Rachel Tice walking by)
Producer: Ughh, spoke too soon. Man, I'm just barfing all day today.
In RWBY's "Heroes and Monsters" episode, Roman Torchwick gives a motive rant to Ruby about how she should die if she wants to be a hero, and how he'll do what he always does: lie, cheat, steal and survive. He gets rewarded for this speech by being made into Griffonfood.
Even earlier in "Fall", when Yang is going to take on Mercury in the tournament, Ruby, Blake and Weiss are watching from the stands and Ruby shouts "Break a leg, sis!" down to Yang. Not long afterwards◊, Yang is actually suckered into "breaking his leg" on live TV as part of the villain's plan and ends up getting disgraced and arrested. Ouch.
In a Garry's Mod video called "Sniper Goes To The Opera", a woman says to her kid "And that's why, little Billy, you can never play in the street. You might get yourself killed out here", while both are standing in the middle of the road. She tries desperately to invoke this by pushing the kid closer to the middle and repeating the last sentence a few times, but nothing happens.
DSBT InsaniT: Done a few times, but the best example is when Frog points this trope out...and promptly realizes he fell for it in 'The Party'.
Martha: Koden, this party is already a disaster, it can't get an—
Frog: Don't say it! Whenever you say 'It cant get any worse', IT ALWAYS GETS WOR— Oh no! WHAT HAVE I DONE?!