I contracted out a vinyl re-siding job (I know, I would have done it myself but it was more cost effective and MUCH quicker this way).  I supervised much of the work, ensuring proper lapping of tyvek, flashing the windows as they worked, and repairing any rot they encountered.

However, I forgot to do/ask them/specify what I call a kickout flashing underneath windows.  Essentially, after housewrap and window wrap and all that, just before putting the j- channel around windows, I put a piece of coil stock at each lower corner, beneath the j-channel, which goes from the bottom of the window to overtop of the nailing flange of the first full piece of siding below the window.  With all the rot I found and structural repairs, totally slipped my mind.

So, any thoughts on the importance of this?  Seems to me that without this, any water above the window will automatically be dumped behind the siding below it, to run down the housewrap, as it hits the top j and is channeled to the sides.