.NET 2.0 and Windows 8
Inexplicably .NET 2.0, 3.0 and 3.5 are not installed by default in Windows 8 and can’t be installed using the redistributables that worked with previous versions of Windows. You have to go digging in Windows Features to get anything older than 4.0.
(Related: Going Chrome; Catfood Software Support; WiX Tricks for Screen Savers)
(You might also like: Crushing PNGs in .NET; Sun; Home of the Whatevers)