This is not strictly an SDL Tridion issue, but I figured I would share it here in case others ran into the problem.
I just installed a hotfix on my development machine for a publish states issue in SDL Tridion 2011 SP1 which fixed a problem on marking items as published or not after installing the service pack. To apply the hotfix I needed to install an updated Tridion.ContentManager.Publishing.dll into the GAC.
I downloaded the hotfix to a share on the development server and followed the instructions in the release notes, and all went well. However when I performed the same operation on our production server (having copied the DLL from the share on our development server) our CMS stopped working and threw the following error:
An attempt was made to load an assembly from a network location which would have caused the assembly to be sandboxed in previous versions of the .NET Framework. This release of the .NET Framework does not enable CAS policy by default, so this load may be dangerous. If this load is not intended to sandbox the assembly, please enable the loadFromRemoteSources switch. See http://go.microsoft.com/fwlink/?LinkId=155569 for more information.
I had forgotten that when copying from a network location, you sometimes need to ‘unblock’ the file by right clicking on the DLL, going to properties, and then ‘unblock’. After unblocking the DLL and restarting the IIS, the Tridion Services and Tridion COM+ application all was well again. Hope this helps if you run into this error.