Technologies or Scientific discoveries that could have been made long before (or after) they actually were.

[Fulano]

Seriously, it astounds me how long it took for people in the Middle Ages up to the 19th century to find out Miasma Theory was false

I think one might be able to disprove the miasma theory and create something better but not prove the germ theory.

The germ theory per se might be impossible, but there are definitely better ideas that people could have had.

Miasma Theory could have been wrong, but at least was not as bad as the Humoral Theory.

In other things, I have always thought that the Roman Numerals seriously hindered any chance for the Romans to advance in a lot of fields that depended on advanced math. I mean, even doing the most basic math with those are a pain in the ass in comparison with the Indo-Arabic Numerals.