In Hookie Dookie Panic, Adda pulls a few of these, the biggest being when she loses a Dance Dance Revolution tournament due to a fight going on, and she proceeds to go into a psychotic rampage and kill those responsible for the fight.
Few days ago, in Sam & Fuzzy, Mr. Blank had his moment of Unstoppable Rage against Mr. Black, at least in the beginning. The funny (or creepy) thing is, the audience could already sense the doom months ago.
Averted in Sluggy Freelance. Torg appears to go into one of these after Horribus kills Alt-Zoe, but, in a rare moment of rationality, he realizes saving the world is more important than a grudge match, and runs away instead.
Girl Genius has this for its Sparks. When they get in the Mad Mood, they unleash all their most devastating gadgetry and can throw around clanks single-handedly. However, it is said that this is also the reason for most lower level sparks' deaths. They get in a rage without the ability or sense to keep themselves from dying.
Tarvek flies into one of these while fighting Zola, beating her to the point where she tries to surrender - only Tarvek isn't listening to silly notions like that.
Agatha goes full tiltafter Lars is killed. Thank goodness for Sparky pies.
Dr. Jean Poule in The Inexplicable Adventures of Bob! can go from adorable Girl Next Door innocence to blind fury without too much trouble. This rarely manifests as physical violence, as the mere force of her yelling is usually enough to produce results (i.e., Bob: "Wow! Thanks, Jean! You're swell at yelling at people!" Jean: "Well, the secret is to enjoy it...") but it has gotten her into fights upon occasion.
In Slightly Damned, Buwaro was born with permanent Unstoppable Rage. The only reason he's so happy-go-lucky is because his star pendant keeps the rage in check.
Karkat Vantas walks around in a state of perpetual ire and will verbally tear into anyone with little provocation, so it's kind of a subversion that he doesn't do this.
Gamzee Makara's title is the Bard of Rage. From all accounts, it manifests as him being nigh-unstoppable when he gets going - he is said to have dealt the most damage to the Black King with just one strike, and that was when he was high. Whenhe'ssober, this trope is more or less his default state. To wit: what he did to Nepeta...
Eventually however, Gamzee's rage is stopped by Karkat via a Cooldown Hug.
Bec Noir goes into this after Jade dies again, and slaughters most of the Trolls' Dream Selves in their session.