Yeah! I thought I'd seen every episode of TOS, but I must be in for a treat because I still have to see the one where Spock wears that helmet.
Deleted scenes from "Spock's Brain" where McCoy discovers that he forgot to reconnect Spock's dignity nerve. The first officer wore it while riding a tricycle around the corridors before security snagged him with a butterfly net.
In HD, the helmet can be seen in the background of FIREFLY episodes along with the Han Solo in carbonite model...