What're some of the most insane things that happened in an episode of Trek and then never got mentioned again?

Such as some crazy technology that was discovered and then forgotten about, or something utterly traumatizing that happened to a main character that they promptly forgot about by the next episode. Or just something super weird and off the wall that happened one time that you're surprised no one ever mentioned again.

One example that I thought of - the fact that there was a giant clone of Spock just out there somewhere, doing who knows what. And he was never heard from again (although LD revealed what became of his remains).