Worldsize -- same reason why WotC Magic the Gathering playtesters didn't recognize the threat of the Mox Gems, Ancestral Recall, Time walk, etc. until the game had been released to the general public. In the case of Haste, for instance, it was that, among 3 designers, 20 editors, and 1000 playtesters, no one realized the real power of an extra standard action until about a year after its release. Now, before I see a half-dozen replies of, "3E Haste doesn't unbalance my game," I accept that as perfectly valid - but I also accept as perfectly valid that Haste created a problem for the majority of people that WotC heard complaints from between 2001 and 2003. Its why the only way to TAKE a full extra action is Quicken Spell, now, as highly priced as it is.
Just like a battle plan never survives intact from first contact with the enemy, a game, computer program, manual, etc. never survives public scrutiny without flaw.