Both GetService() and GetServices() state in their documentation "Must never throw":

    /// Gets an instance of the given <paramref name="serviceType"/>. Must never throw.

This seems inaccurate, there are a few situations in which throwing from one of these methods might be appropriate:

• ArgumentNull - if the serviceType parameter is null then the resolver should probably throw
• Misconfigured dependency resolver - e.g. if the serviceType is available but can't be constructed; this is a fail-fast that helps catch configuration issues
• OutOfMemory/StackOverflow/AccessViolationException etc. (runtime exceptions)
  

The full intent of the statement is probably related to the returning null/empty in the case of being unable to service the request, so I would suggest: "Should not throw when a service is unavailable".

Minor related item:

GetServices() documentation includes the following misspelling of 'instances':

    /// <returns>A sequence of isntances of the requested <paramref name="serviceType"/>. The sequence