Those still using Autodesk® Revit® products from the 2012 and 2013 releases but who have installed a 2014 or later release product from Autodesk or have in some other way had .NET 4.5 installed on their computer will find that this creates issues with the contextual ribbons in Revit. Due to the recent overhaul of the Autodesk website and the introduction of the Autodesk Knowledge Network, old links to the required updates (2012 & 2013) and hotfix (2012 only) may no longer work. To save others from the effort to search for the reuqired files, here are the current URL's:
2012 - Make sure you have Update Release 2 for your product installed before applying the hotfix.
Revit Architecture 2012 Update Release 2
Revit MEP Update Release 2
Revit Structure Update Release 2
Hotfix - Autodesk Revit 2012 - Ribbon behaviour due to .Net 4.5 installation
2013 - Install the correct version of Update Release 3 for your product. Note that the "one box" version that ships with the Building Design Suites has a different update file than the Revit Architecture, Revit MEP or Revit Structure versions.
Revit Architecture 2013 Update Release 3
Revit MEP 2013 Update Release 3
Revit Structure 2013 Update Release 3
Revit "One Box" (Building Design Suite) 2013 Update Release 3