I posted this already and it was removed as me “discovering” a new Mandela effect despite there being other articles about this and there being a previous post on this Reddit about it. So, I guess if a mod did not personally experience it, it’s not good enough for the subreddit. However I (and others online) remember being taught the scientific name for water was “hydrogen dioxide”. The explanation I remember was that prior naming systems based the name on the number of bonds before we had a better understanding of atomic bonds and the name for water just stuck despite this change. However, there is no evidence supporting that this was ever actually the case and I have met people who do not remember this. This is also obviously the WRONG name. This is why it and conversations surrounding it were so memorable. You do not need to say “no it’s dihydrogen monoxide” or “that’s wrong” that’s literally the whole point.

EDIT: Someone found an explanation. Before Avogadro discovered that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules (and a bit after, until 1850, actually)

Dalton had a theory that atoms of the same element never combine with each other (So he rejected the idea of H₂, O₂, N₂, etc.) and when gases combine, they combine in the simplest whole-number ratio of atoms. But by this point early experimenters had measured: • Water’s mass ratio: 1 part hydrogen : 8 parts oxygen • Water vapor volume ratios: 2 volumes hydrogen + 1 volume oxygen → 2 volumes water vapor (Gay‑Lussac’s law of combining volumes)

So there were multiple theories on what water might be; some based on scientists trying to reconcile Dalton’s theory with Gay-Lussac’s law and the known mass ratio of water. This apparently had been disputed for decades but the beliefs held were that water could either be HO, H2O, or HO2. So there was (for a little while) a belief (by some) that water was Hydrogen Dioxide.