Fallout:New Vegas. I modded it with a couple companion mods, created a female lead, then proceeded to have my 3-woman squad dish out carnage and death to the wastelands in stylish cocktail dresses. Who needs armor, when you can kill your opponents in a lovely evening dress?

I'm not sure about anything truly absurd (though I'm sure I probably did some weird stuff in Hitman) buy when I played Oblivion I took a quest in the coastal town (there was only one as far as I remember) about a haunted ship, thinking my silver dagger would be enough to slay the ghost. It wasn't. I ran away (or quick teleportes or something) and found that the ghost had escaped the ship into this town and slaughtered most of the inhabitants, spoiling a whole host of quest lines if I remember rightly. Can't remember if I had a save I could reload before it happened or if that was the point I finally gave up on the game...

Flying the tank around in GTA 3 was also pretty absurd...

I have very pragmatic approach, which makes me do seemingly absurd things in games, for example:

1. Baldur's Gate I - adding.Xzar and Montaron in party, stripping them from all valuables, feeding them to the local wolves, then kicking from party. Voila! Free gold and items. Rinse and repeat with other potential party members.
2. Evil Islands - adding Hador in party, stripping him naked, then throwing him to local boars. Rinse and repeat with all the other party members.
3. Wizards & Warriors - I keep recruiting new heroes, taking all their gear and gold and kicking them out. Rinse and repeat until I have enough gold to last me for some time. So many potential heroes were robbed by me, I could win the title of main villain of the game.

Playing Fallout 2 for the x-time I transformed it into a real destopia by killing every NPC.

Pokemon Mystery Dungeon:

I got a Smeargle to learn Mind Reader (next attack guaranteed to hit) and Perish Song (attempts to inflict Perish Song (target dies in 3 turns) on every enemy on the *floor* (notice, *floor*, not room), but has low accuracy). Then, if I use the two attacks in sequence, I can wipe out all the enemies on the entire floor. I think that is rather ridiculous. (Unfortunately, you don't get any XP for this.)

Final Fantasy 4 Advance:

I played through the endgame and bonus dungeon with a party of Cecil, Rydia, Rosa, and the twins. (In other words, Cecil was the only one who isn't a primary caster.) If I could, I would have switched Cecil with FuSoYa, but unfortunately, that's not an option for 2 reasons.

Final Fantasy 5:

On one playthrough, on reaching Omega, I killed it before continuing, and without excess powerleveling.

Wizardry Gaiden 3:

I ran into what I consider to be a design flaw, so I cheated to raise a thief's level to 33 or so just to unlock doors that were in my way. (I was using a bard in my party, and bards, while otherwise able to replace thieves in this game, are unable to open those locked doors at reasonable levels.)

Centauri Alliance:

There is a rather serious bug that is triggered by using psionic items in battle. I was able to use the bug to get me items that could do things like restore PSI fully with infinite uses, or get unlimited casts of CHAOS (a very powerful offensive power) early in the game. Furthermore, I actually, with the help of a debugger and a hex editor, managed to fix that bug without access to the source code.

I break every window in shooters, the more satisfying the sound design and visual feedback the better.

Waaaaaaaaay back when I used to play warcraft, I used to have my lowbie Shadow Priest (around level 9 when first got there if I recall) sat fishing in Iron Forge, where the only other time I would leave, would be to enter a battleground.

Whilst sat in Iron Forge, I would let all others know (via /trade, so all cities saw the message), that if anyone wanted anything enchanted, I would do it for free.
All they had to do, was come visit me.

The part that was nutty?

I was an Undead (aka Horde) Shadow priest, sat in an Alliance city, surrounded by anything from other lowbies to guards & level 85s that could one shot me with damage to spare.

lol, I used to do that with ants in SimAnt by using rocks to trap them.

For me, it was playing a game of Neverwinter Nights on a server running a mod called "Legends of Chance". Without being too careful about reading the forums, I joined their server and erroneously went from the lowest-level "goblin woods" precariously exploring into a new area of the map.

I entered a very serene wooded area, thinking I was going on a hike, when all of the sudden a group of "Wemics" (they had the heads of lions, the torso of men and the bodies of horses) showed up and started attacking.

After one hit, I was near death, so I went into a panic, let adrenaline start pumping, hair standing up on neck, I did the first thing anyone in my position would do. I ran. I don't know how, but it was like my feet grew wings... or maybe it was that I was just two steps of hitting the trigger that took me back to the goblin woods.

I appeared in the goblin woods again, watching others running around herding, trying to calm myself down worrying how I was going to get past the next goblin that I spawned... so much for a break.

Next thing I know, one of the Wemics appeared right next to me, as if they continued to chase me from their 'some'teenth level area into the area where all the noobs (or at least new characters) where trying to herd goblins and level up. "Fright and flight" instinct kicked in again and I just started running towards the store in the middle of the goblin forest, as I was running I saw another... and another...

The Wemics I spawned just kept pouring in, targeting not only me, but any other player who happened to be in the goblin forest indiscriminately and with contempt. I got halfway between the edge of the forest where I brought those things in and the store in the middle of the forest before one galloped up my back as if I were a launching ramp. Being already on deaths door, that was the knell of the bell of doom.

There I was, my character face-planted in the ground, while the Wemic that tromped me galloped on, searching for its next victim. For the next five, maybe ten minutes I laid there, not wanting to spawn again and lose valuable experience, waiting for something to hopefully come and heal me. But that wasn't to happen any time soon.

During that time, I watched as other players ran back and forth the area, running right past my prone, dead body, commenting about the "Wemic Invasion" and "Whomever brought the Wemics here was 'most unkind'". Characters ran, Wemics galloped happily back and forth along my view, laying waste to anyone who dared enter the goblin woods. I saw fellow players fall before me, our corpses staring at one another as the random Wemic would gallop on by sometimes chasing a bewildered player who got a nasty surprise on game entry.

I felt bad, I was ashamed, I should have really read all the forum posts before playing, but all that time my character was lying there dead and I was watching the carnage take place and seeing what everyone was saying, I couldn't help but laugh myself silly.

Did you eat The Pickles(tm)?

In Syndicate, I'd lay waste to each map, destroying all life using lasers, miniguns, or gauss guns. With so many people to incinerate, it actually took a while on some of the more populated maps.

Good laugh, thanks for that +1

I always love unintentionally trolling large numbers of people in multiplayer games. It makes me laugh so much more harder then when I intentionally try.

It was entirely unintentional, but watching those things gallop back and forth chasing people in and out of view and some of the comments people were making just made me laugh. In the end, there were probably about three others around me laying prone on the ground waiting for someone to come along an heal, fortunately, a higher level character ported in and cleaned them out.

It was just a comedy of my error. :)

I pee'd green sick pee on my dog and made him throw-up and eat it in Postal 2 :(

Oh i also made a spell in Oblivion that stripped off everyone's clothes, armor and weapons enraged them and made them all fight each other in the entire loaded cell... I called it "Party hard"

I also bought Spacesiege.... I'm so sorry :(

On Final Fantasy VII I mastered every materia 8 times, so each character had master magic, master summon and master command. Then had some spare materia so I could setup myself for killing Ruby Weapon (final attack materia + life because fire heals Ruby, etc etc), then the descent into the crater to kill Jenova and Sephiroth.

I played FFVII a hell of a lot, sometimes setting challenges like leveling to 99 before finishing the first reactor.