From Noonan's recent WotC web column:
http://www.wizards.com/default.asp?x=dnd/dd/20060519a
... But often, either the players or the DM will try something, see it work in a completely awesome, game-breaking way, then never use it again.
When we were playtesting Eberron, I had a player who came up with a completely abusive combination of mechanical artifacts including the shifter race, the weretouched master prestige class, and the warshaper prestige class from Complete Warrior. When he’d accumulated all the pieces of his combo, he tore through everything in his path with no difficulty. Later, he came to me and said he wanted to respec his character out of warshaper and change a few feats.
...
What motivated the player to “downshift” his shifter? Partly an innate sense of fair play, I’m sure. And in D&D I think there’s an implicit promise to not overshadow the other players at the table—to give all your buddies their moment in the spotlight.
Where did he get these players?!?
Everytime I get someone who find a broken combo, they ride it for as long as they can. Some even whine that's "not fair" when I put a stop to it. They expect me to "meet the challenge" with NPCs built just to tackle the problem PC instead of putting them back on track with the other players.
Ironically, some of these guys finally understood, when they started GMing.