Playing around with WiX 3.0, candle.exe started emitting a weird message and failing:

   WixUIExtension.dll could not be loaded

I fixed this by editing the WiX project file by hand and changing the HintPath to remove the ".dll" extension, i.e. instead of this:

   <HintPath>../path/WixUIExtension.dll</HintPath>

Use this:

   <HintPath>../path/WixUIExtension</HintPath>