Fatal error CS1647 on one server but not other

This error reared it’s ugly head one day and we were very perplexed as to why. The code was running just fine on the development server but on production we were receiving CS1647: An expression is too long or complex to compile.

After much digging as to why this might be, it was discovered that the .NET versions on the servers were different, but not dramatically so. The development server was running 4.6.2 while the production server was 4.6.1. A simple update to the .NET version and the error was resolved.