I know this sounds improbable, but after installing Revit Architecture 2014, My Revit 2012 and 2013 Ribbon Menus are messed up.
The options on the ribbon for any selected object are either wrong or missing altogether. I have to cycle through all the ribbon menu displays (little button at the top right) until I"m back to the full menu display, which then looks correct, but after selecting edit sketch, or anything else, the menu is screwed up again and I have to cycle through the ribbon display options again for it to display correctly.
In sketch mode, the menu items are of the wrong category, and Revit appears confused about what mode it's in. Just selecting a stair or floor, puts it half into sketch mode, with the wrong set of tools available. The family editor has the same problem, offering "edit family" on the ribbon when a simple extrusion is selected. Again, cycling through the ribbon menu display types will bring up the correct menu, which works for one click, and then you have to cycle through another 4 clicks of the ribbon menu options again.
Uninstalling Revit 2014 doesn't fix it, and running a repair on 2012 doesn't fix it, and a re-install of 2012 doesn't fix it. I'm now trying an uninstall and re-install of 2012.
Some of the URL's in the earlier posts in this thread are no longer valid, due to the recent overhaul of the Autodesk website and the introduction of the Autodesk Knowledge Network. Here are the current URL's related to the contextual ribbon issues in the 2012 and 2013 versions of various flavors of Revit when .NET 4.5 is installed:
2012 - Make sure you have Update Release 2 for your product installed before applying the hotfix.
Revit Architecture 2012 Update Release 2
http://knowledge.autodesk.com/support/revit-products/downloads/caas/downloads/content/update-2-for-r...
Revit MEP Update Release 2
http://knowledge.autodesk.com/support/revit-mep/downloads/caas/downloads/content/autodesk-C2-AE-revi...
Revit Structure Update Release 2
http://knowledge.autodesk.com/support/revit-structure/downloads/caas/downloads/content/autodesk-C2-A...
Hotfix - Autodesk Revit 2012 - Ribbon behaviour due to .Net 4.5 installation
http://knowledge.autodesk.com/support/revit-products/downloads/caas/downloads/content/hotfix-autodes...
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
http://knowledge.autodesk.com/support/revit-architecture/downloads/caas/downloads/content/revit-arch...
Revit MEP 2013 Update Release 3
http://knowledge.autodesk.com/support/revit-mep/downloads/caas/downloads/content/revit-mep-2013-upda...
Revit Structure 2013 Update Release 3
http://knowledge.autodesk.com/support/revit-structure/downloads/caas/downloads/content/revit-structu...
Revit "One Box" (Building Design Suite) 2013 Update Release 3
http://knowledge.autodesk.com/support/revit-products/downloads/caas/downloads/content/revit-2013-upd...
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