When are interfaces for classes technically necessary with Spring?

Someone told me, that he learned in a Spring training, that whereas directly using classes without interfaces would work in most cases with Spring it is actually not guaranteed.

For me is clear, that if I use spring to integrate with another framework and that framework requires an interface than of course I have to provide an interface (e.g. for remote EJBs or webservice frameworks)

But are there any cases where the Spring IoC Container or Spring AOP technically requires interfaces? (I'm talking not about good design and best practices here, that's another discussion)

Or are there any other parts of Spring, where direct usage of classes without interfaces currently works by some kind of coincidence but is actually not a designed feature and may not work with future versions any more?

Thank you in advance for clarification.

Kind regards,
Rick