My favourite one is that Mercury is a liquid at room temperature. It should be a solid, it doesn't form diatomic molecules in gas form, and it has far lower mel

My favourite one is that Mercury is a liquid at room temperature. It should be a solid, it doesn't form diatomic molecules in gas form, and it has far lower melting points and electrical conductivity; largely because it doesn't share it's valence electrons easily due to a mixture of relativity and quantum effects.