Phil Leeds appeared in three different episodes - You Can Go Home Again (#4.1), Quiz Show (#4.19), and How to Succeed in Business (#5.9) - twice as an old man who appears to have been lured to a restaurant for years.
Rider Strong (Shawn Hunter) can be seen with an earring in his upper left ear. It remains for only a handful of episodes as the earring can't be taken out for a few months until it heals. It is removed for filming by the start of season 5.
Steven Gilborn appeared on The Wonder Years 3.2, starring Fred Savage.
Due to the moon's orbit, it does not weigh anything. However, the answer that the students from Einstein Academy provided (81 quintillion tons) is correct was to be placed on Earth before it collapsed under its weight.