Today I learned that early versions of the periodic table included a material called didymium as an element. It was later discovered that didymium was actually an alloy of praseodymium and neodymium.

https://en.wikipedia.org/wiki/Didymium

https://www.reddit.com/r/todayilearned/comments/15kbh2z/today_i_learned_that_early_versions_of_the/