Each player can't cast more than one spell each turn.
10/4/2004 When Arcane Laboratory enters the battlefield, it will look back over the turn to see if any spell was cast before it entered the battlefield. It will notice itself being cast.
10/4/2004 If a spell is countered, it still counts as the one spell cast.
10/4/2004 When it leaves the battlefield, the effect ends and all players are able to cast spells unrestricted again.