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Daughter: What? No, you couldn't have bleegle warble yargle arrgh!
Bob and George, where Future Alternate Mega Man (it's a long story) fights Bob during the events of Mega Man 5. Mega Man, using Gyro Man's powers, finds a way to penetrate Bob's supposedly indestructible flame barrier. Bob is very angry about this, but says nothing; Mega Man calls him on it, suggesting he's "inarticulate with rage."
PS238 has a character implanted with a chip which replaces any profanities he voices with random other words. If he tries to launch into a Cluster F-Bomb, he instead starts singing showtunes.
8-Bit Theater makes this a Running Gag, primarily with Black Mage although others have fallen victim to it. It's usually caused by something Fighter says or does.
Black Mage: Ffffffffffffffff Fighter: I think he sprung a leak.
Also one strip later,
Black Mage: Arghble!
And after Black Belt climbs across a rope he was holding himself.
Black Mage: How... you... here... what?
Black Belt: I held the rope up and walked across just like you guys.
Black Mage: That doesn't. I mean, you can't.
Red Mage has one, too:
Fighter: Don't you understand? With gravity slain, we can now fly! * flies* Thief: Huh. Red Mage: But he. You can't. Love, hate, clouds. *falls over*
All of trolls in Homestuck have a particular pattern in the way they type in chat logs, and Vriska in particular uses the number 8 in place of "B" and homophones of 8. However, she tends to go a bit overboard with their use when she's flustered.
AG: Or you know, if you're so h8gh 8nd might8 an8 th8nk you're so gr8at, m8y88 you c8uld oh I d8n't kn8w........
AG: TRY AND ST8P ME FROM DO8NG B8D THINGS????????
During Act 6 she moves into the cat-stepping-on-keyboard mode, responding to one of Hussie's barbs with "sdkljfhkldhfsdkhfk".
And Karkat. Although given that he's an angry alien troll it's difficult to tell if he's using actual troll insults or not. Here's a gem:
CG: YOU ARE SUCH AN IGNORAMUS I COULD SHIT MILES OF RAGE SNAKE TO CHOKE YOU TO DEATH.
In Penny and Aggie, Aggie publicly accuses Penny of blowing off a date with Duane "because he's Black." When Penny asks her to repeat that, Aggie, realizing the enormity of what she's just said, falters for a moment ("Because he's... Because... you're not...") but then comes back at her with "BECAUSE HE'S BLACK! BECAUSE YOU'D NEVER BE SEEN WITH A BLACK MAN! YOU'RE TOO GOOD FOR HIM TOO GOOD FOR ANYONE WITH YOUR BLONDE LITTLE BLONDEYOU'RE WORSE THAN HITLER!"
In The Order of the Stick, when Haley's looted dragon treasure got blown up, she actually becameaphasic, and spent a whole arc speaking gibberish. In fairness, it did turn out that the treasure thing wasn't the root cause.
When Nale realizes the sign in Girard's Pyramid saying that the Gate is in another pyramid was a ruse, and that the Order beat them to the Gate (and destroyed it along with the Pyramid), he is temporarily stricken with this. "I can't believe- how did he- so the Gate was in the- GREENHILT!!"
In El Goonish Shive, Justin gets so angry with Elliot in this strip that he is unable to properly respond to Elliot's last comment. He is, however, in control of his emotions enough to tell Elliot he can't continue the conversation, make one last statement and say bye.
While Coach Z of Homestar Runner speaks somewhat incoherently by default, getting frustrated while trying to correct his speech only makes it worse.
In General Protection Fault, Ki's mother, a Chinese woman married to a Japanese man and living in America, calls up Ki to tell her about her father being hit by a car and begins talking in a mixture of English, Chinese and Japanese. Her dialogue is represented by "*@#&!"
This strip from Ctrl+Alt+Del is liable to qualify for this trope as well, when Ethan dissolves into incoherence at the notion of using his obnoxious colleague Rob's 3D models in his video game project.
Another example occurs after Ethan discovers that his title of "King of Winter-een-mas" has been usurped.
In Cat Nine Insulting her dad can make Myan angry enough for this. 
In Schlock Mercenary,when Elf finds out that Petey had been spying on her boyfriend's room (along with everyone else on the ship), she starts growling gibberish, causing Kevyn to tell her that he doesn't need her to get incoherently angry on his behalf. Elf stresses that Petey watched EVERYTHING that occurred in the room, which Petey confirms "It's true." Kevyn responds "You... My... SEX... WATCHED?! Petey says it's also true that Elf doesn't need to get incoherently angry on Kevyn's behalf.