I was recently working on an upgrade to a website that involved changing the Application Pools from 32-bit to 64-bit. The servers hosted several websites and they all ran in 32-bit mode. Several days later I noticed the System log was full of the following errors for one of the websites that I didn’t update.

Parser Error Message: Compiler executable file csc.exe cannot be found. I recently ran into this error when upgrading an application from ASP.NET 2.0 to ASP.NET 3.5 on IIS7.5.