@ProNotion is right, I have seen more and more packages on Umbraco relying on specific versions of assemblies causing assemblies to conflict especially if two packages need different versions of the same assembly.
So some other tips:
This tool from MS helps diagnose assembly binding problems (http://msdn.microsoft.com/en-us/library/e74a18c4%28v=vs.71%29.aspx).
Adding a runtime / assemblyBinding section to your web.config can work round problems with clashing assemblies (http://msdn.microsoft.com/en-us/library/0ash1ksb(v=vs.110).aspx)
Sometime the assembly doesn't get copied (eg you use msbuild and the assembly isn't included in the project) - so you should also check to see if the assembly made it to the live server.