So what happens, you write some C# code to go with your WIX installer. The C# code only runs on uninstallation. But ... during development you write some dodgy code, it causes an exception which makes the uninstallation fail.
Take a look here,
I found that msizap didn't work for me, but what did was going to the directory '%windir%\installer' and searching for the most recent file, editing it with orca, and removing the custom action causing a problem did the trick nicely.