I unintentionally broke a 3E game I ran a few years back. The players were complaining that there weren't any worthwhile feats, so I introduced a feat from Star Wars d20: Heroic Surge -- this gives the PC an extra standard action, once per day for every 4 character levels (so, a 13th level PC can do it 4 times).
The arcane spellcasters in the party jumped all over this (and, for that matter, Haste, which was busted in 3E, as someone else has already pointed out). It became not uncommon for the two arcanists to pop off a total of 4 big-damage spells at the beginning of a combat, and that would pretty much finish off the bad guys. Made it hard for me to write, and it also ticked off the non-spellcasters, since Heroic Surge was good for them, but not *that* good.