With uninstall custom actions you have to be really careful.

#1 Rule

When working with uninstall custom actions, always encapsulate the custom action in a try / catch block!

Why?

If you are testing your uninstall process and it throws an exception you will be left with a half uninstalled application.
As the application has not un-installed you cannot install a new version and as the uninstaller is compiled you cannot rebuild the installer to fix the error that is causing the exception.

How do I know this?

I did this exact thing. I was creating an installer that had custom actions for both install and uninstall.
I caused an exception in the uninstall process and I was left with a product that could not be uninstalled!

Can you recover from this?

I found that in this scneario the Microsoft Fix-it tool is your saviour.

Download the fix it program from here, Microsoft Fix-it Uninstaller

Run the program and follow the prompts.

I did a custom uninstall, chose my product that had failed to uninstall and it cleans up all the relevant registry keys so that it has been completely removed from the computer.

Cheers