When a nice, normal character suddenly goes into a rage, you've hit his Berserk Button.
Old World of Darkness - If you ever cross paths with a member of the Sabbat on the Path of Honourable Accord, don't ever, ever, ever, EVER accuse them of breaking their word. They will pursue you with the full fury of a raging vampire until you either apologize, die, or kill them.
Taken almost-literally by the Arco-Flagellants in Warhammer 40,000, which are designed specifically to turn into berserk killing machines with the proper command. Don't examine them too closely, either. Bring up the Squats at any GW event, and you'll find that the Berserk Button works in real life, too.
Mentioning the horrible fanfic Squad Broken is a general berserk button for the fan base.
So anyway, what happened to the Squats? *duck and cover*
Eldar: In peace, they're gentle, logical, at-one-with nature, etc, with peaceful, quiet occupations. In war, they turn into furious killing machines, losing their identity to the overwhelming desire to annihilate the enemy.
Suggesting harming civilians in front of the Salamanders is a bad idea, even if you're a chapter master.
Also concerning the Tau, Gue'vasa auxiliaries are a Berserk Button for the Imperium. So much so, that Imperial units get a close combat bonus against them.
GURPS has multiple was of simulating this on a character, the most comon of which is simply called Berserk
The "Consumer On the Brink" archetype from the Feng Shui supplement "Seed of the New Flesh" is not very powerful in terms of skills. But attack him or put him in close proximity to his irritant, which is a Berserk Button or something else that pisses him off to no end, and he gets 4 bonus points that he can distribute however he wishes between his Body and Reflexes stat, which can affect his combat AVs.
To make lycanthropy less predictable, Ravenloft encourages Game Masters to link werebeasts' involuntary transformations (and subsequent Berserk Button rampages) to phenomena other than the full moon, such as the tides, exposure to a particular odor, food or substance, or stressful situations such as feeling hemmed in by a crowd.
In Paranoia, avoid being a Communist. Friend Computer will allow mutants to register their powers, and may even be lenient on traitors. But if you are a Commie (or made to look like one), you WILL be terminated.
While Friend Computer is merciful to those who were mutated by foul Commie sabotage of their cloning tanks, registering your Machine Empathy mutation is not recommended, as it will not only get you terminated, but ERASED as well.
In Unknown Armies, the Rage Passion that all characters have is that thing that really sends a character into pissed-off rage mode. As Passions are perhaps the deepest part of an Unknown Armies character and the character goes all spooky-intense when put into situations that trigger one of the Passions, the Rage Passion is very much a dramatic Berserk Button.
Exalted: Even if an Exalt fills their Limit meter, it's not necessarily enough for them to go into a Limit Break. Rather, each Break comes with a trigger condition, such as "Annoyed at the childish nature of the world" or "Witness an act of suffering that you could easily stop." A good example of this trope in its pure form is the Red Rage of Compassion break, which typically involves a normally kind-hearted and compassionate Solar going into an Unstoppable Rage after seeing an innocent cut down.
The Demon prince Graz'zt is probably the least likely demon lord to rip you apart just for existing, even opening his layers of the abyss for trade. Mention that he actually fell in love with Iggwilv, that Malcanthet turned him down, or call his domain "The Little Hells", and he will feed you to the predatory plants in his gardens.
Of the various Clans in BattleTech, the Ghost Bears are frequently considered to be the most laid back and least aggressive. Unless you attack their capital, Alshain. Or use nuclear weapons. Or they have any reason whatsoever to even suspect that you might be associated with Clan Wolverine. Doing any of the above makes them angry. That's very, very bad.
While the Diamond Sharks would rather sell you weapons than shoot you with them if you attack their mercantile interests or civilians you are in for a beating. When the Draconis Combine was trying to aquire advanced technology after most of their production centers were destroyed in the Jihad, they went to the Sharks. The local warlord gave them a continent to setup shop but a few of his senior officers took offense to this "Clan occupation" and killed a few hundred civilians. By the time the warlord managed to call his subordinates off the Combine had lost two regiments.
