If the project being tested references Autofac 6.40 then a test project using FluentAssertions.Autofac 6.5.1 will fail.
But it just needs a redirect in the app.config:
<runtime>
	<assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
		<dependentAssembly>
			<assemblyIdentity name="Autofac" publicKeyToken="17863af14b0044da" culture="neutral" />
			<bindingRedirect oldVersion="6.3.0.0" newVersion="6.4.0.0" />
		</dependentAssembly>
	</assemblyBinding>
</runtime>
So this can be closed