When deploying a solution to a site you can sometimes get errors that are not picked up during compilation.  For example, we created a Content Type that contained a field with an inccorrect BaseType of "Person" rather than "User".  This compiled just fine but fails on the first deploy.  It then failed to compile after deploying.

To fix this is easy enough - remember to update the content type xml as well as the generated schema.xml - but Visual Studio can get it's nickers in a knot and will not be able to deploy the corrected solution.

You need to manually remove the solution using the setup.bat that is created during deployment. Run setup.bat from the bin\debug or bin\release folder thus: 

    setup /u

Then IISReset.

If you try to redeploy with Visual Studio you may now get an error that talks about an "invariant language"  - sorry, I didn't save this error message when it happened.  You will need to restart Visual Studio to clear this error.