I once carefully worldbuilt a computer system, how it would have a distributed filesystem with redundant backups, a public key system for identity, etc. etc.
Then when I was building the next world, I realised I could just copypaste the computer system from the last one. So there's a cyberpunk world and a futuristic democracy, but they have identical tech, in quite a bit of detail.
Another example is a coilgun I figured out, that would fire cheap steel flechettes, with a certain amount of kinetic energy, a certain amount of recoil, etc. The exact same weapon exists in several of my worlds now.
What things do you reuse like this?