Character mistake: When Roy and Johnny are waiting outside the radioactive lab, Roy sends Marco and Mike to replace Chet and the captain. When Chet and the captain come out, the captain points to Chet and then points up the hallway, while he says "All right, take off you two," even though it's only Chet who's heading outside to be decontaminated, and he could not have meant, "All right, take off you too," for the same reason.
Inventions - S3-E22
Character mistake: Dr. Brackett calls Dr. Early "Jim." In the show, Dr. Early's first name is "Joe."
Smoke Eater - S4-E16
Character mistake: When having dinner Chet asks Marco to pass the salt. Marco hands him a bottle of ketchup instead.
Answer: I believe you are speaking of the old man, Mr. Wilson. He's played by J. Pat O'Malley.
Bishop73
Correct, it was J. Pat O'Malley...he also played the grandfather with his grandson when their rocket exploded, and also played "Old Bill" in the episode with Ann Prentiss, where Gage saves the little girl from the burning tree house, and her mother falls in love with him.