How can this happen? And How can I fix it?

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My guess (and I stress it is just a guess) is that the test assembly and the assembly under test either reference different versions of the
System.Web.Mvcassembly, or one is ultimately resolving a different version. Either way, ultimately the test assembly is expecting one version, and the assembly under test is returning another.Try doing a clean then a full rebuild, and check other culprits like the web.config binding redirects as @SLaks points out in the comments.