Out of the blue, Visual Studio 2013 (Update 2) refused to debug ASP.NET 3.5 Web site projects. The error message was:
JIT Compiler encountered an internal limitation.
My ASPInsider colleague Jeff Fritz helped me confirm that it had nothing to do with the controls in the site - the test project ran fine for him.
Repairing VS 2013 didn't work. Restoring my system back to an April restore didn't change anything except cause VS to crash with no error message.
Things became more clear when I ran VS is Safe Mode. Instead of the bogus JIT error, the VS debugger complained:
unable to launch the configured visual studio development web server.invalid pointer
At least that gave me something more to go on. Repairing IISExpress didn't work.
Finally, I saw a suggestion to clean up the VS Environment. In VS 2013, I went to Tools > Import and Export Settings > Reset all Settings. I threw away the previous settings file.
BAM! Debugging on the ASP.NET 3.5 web site started working again.
Lost the best part of the work day on this but maybe you won't have to lose yours as well?