The Confirmation Polarization - S12-E13
Corrected entry: Sheldon meets two new fellow scientists, one of which is Sean Astin, who played Sam Gee wise in the Lord of the Rings. Surely, Sheldon would know who he is.
Correction: I'm not sure if this is meant to be a troll entry or a joke. Sean Astin doesn't play himself in this episode, he plays Dr. Pemberton.
Correction: Suspension of disbelief has to play a part here. Actors are allowed to play other roles, and being many years later, Sean Astin looks quite different, so even if there was a meta element of "this person looks like the actor who played Sam", Sheldon wouldn't necessarily even notice the similarity.