I feel bad for missing this thread, as I always enjoy these kinds of topics!
I think the most recent case I've had for a surprise VA appearance in a series was with
Gurren Lagann, where Yuri Lowenthal voiced Simon. It took me around four episodes to figure it out, which shocked me because he is usually one of those recognizable voices. I was thrown off by his higher register, it would seem.
Honorable mention goes to the legendary Charles Martinet as Paarthunax.