I've to two of these situations that come to mind, both from the same player:
1) The PCs are about 15th level, in a massive dungeon. They go for the head on assault, and so they are surrounded by about 10 fairly hardy enemies - or at least the cleric is (battle cleric build, pretty tough, decent fighter) as the mage is shapechanged into a pixie and floating invisibly nearby.
The cleric wades into battle, but the wizard (evoker type, lots of straight damage spells) is getting restless as he doesn't have anything handy to zap the baddies with that won't also get the cleric (lots of statues in the way, etc). The wiz player starts whining...
The cleric's player, always the metagamer, tells him that he should just lay a fireball down, centered right on his character. It will help whittle down the enemies (he's feeling their blades now, even with his uuber munchkined AC, as they are using flanking and aid another) and he can take it, especially with his ring of fire resistance. Wiz player says no, its too dangerous, they argue for a little bit, finally the cleric's player is shouting at the top of his lungs: "Just Do it! I AM INVINCIBLE! Nothin can harm me! (not a direct quote, but quite accurate - also note that the PLAYER was actually screaming this, at my friend's house, at like midnight on a Wednesday, with his parent's sleeping down the hall - luckily they were cool about such things, as long as they weren't too frequent
So the wiz lays down a fireball, with the words: 'maximized is all I got though, I already used my other ones up'. The cleric player seems a little worried, but figures he is still ok. He rolls his save (quite a high bonus as well) - fails agains the evoker's specialized class. Ok, he says "Not to worry, I've still got my ring of fire resistance" - I then inform him it only soaks 5 damage per round... which he knew but didn't seem to remember. Cleric-boy then says "Oh well, guess I'll take some damage then. What's that do, like 30 points or something?" He is informed that the 15th level evoker specialist with his maximized fireball does double that damage, and promptly realizes his buddy just napalmed him to death. He was quite upset, especially as everyone else at the table was bustin' out laughing
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2) This player's next character is a sorcerer. A half-orc sorcerer who specializes in buffing spells. He transforms himself into a fire giant, casts all sorts of buff spells and plays as a fighter, basically. Interesting concept really, but it has its weaknesses, as will be seen...
On to the next dungeon. The baddies know they are coming (this group was never quiet or sneaky - not their style). One of the main henchmen is a decently powerful wizard, and he calls his burly bodyguards as soon as he finds out the party is coming. Half-orc/fire giant sorcerer is on point, comes to the entrance to a large room, sees the bodyguards standing like a guantlet before the wiz-baddie and decides to charge. Combat initiated, he rolls initiative after the wizard and his bodyguards. Player #1 starts pulling out a magic item or something, Big baddie wiz starts casting a spell, bodyguards take defensive stances (obviously readying for something, probably the first PC stupid enough to approach...). all PC spellcasters (most of the party) fail their spellcraft checks to determine the specific spell bieng cast by the baddie. Half-orc/fire giant's turn: he charges.
He charges forward, trying to go straight through the gauntley and take out the evil wiz in one shot (had a decent chance too, actually - with his current condition/weapons/etc). As soon as he gets even with the first 2 bodyguards, his fire-giant self shrinks to a half-orc. His buffs go poof. His oversized sword seems mighty heavy and unwieldy. All 4 bodyguards commence whaling on him, killing him quite quickly.
The evil wiz had cast antimagic field. He figured, I'm fighting mostly spell casters, so if I remove the magic equation then my beefy guard can shred them up. The fact that the character who charged first was based almost entirely on buffs just magnified the outcome (and quite hilariously). We had pretty much all forgotten that the character was a half-orc in the first place (it had been like 15 sessions since he had been anything but his favorite fire-giant shape) - and imagining the look on the character's face (coupled with the actual look on the player's face) was just priceless. We still rib him about this one to this day. In face, I think I'll go do that now.
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