Chloramine

http://www.thefreshloaf.com/handbook/water

"If you’re concerned that the chlorine might interfere with the action of your leavening, the solution is simple: fill a bowl with water and leave it uncovered overnight – the chlorine will dissipate completely."

This problematically neglects the subject of chloramine, which is commonly used in municipal water specifically because it does not dissipate as easily as chlorine.