Why would Development Server be finding a specific assembly in the GAC and IIS not?

Question

I wrote a WCF Data Service, when running it on the Development Server it runs without any issues; however, once I deploy it to IIS it throws this error System.IO.FileNotFoundException. The assembly it can not find is located in the GAC. Why would Development Server be finding a specific assembly and IIS not?

Answer 1

Enabling 32 bit applications on the Application Pool fixed the issue.

Answer 2

Just my guess, but I reckon the assembly is compiled as .net 4 and IIS is running under .net 3.5. Check here.