In GCSE Chemistry (taught to 15-16ish year olds), we were taught that the first shell contained two electrons, and subsequent shells contained 8 (up to the final, which may contain fewer).

Then, after GCSEs, during A-levels, we were told (by the same teachers, in the same classrooms) to forget this model, and that the situation was actually more complicated, with s, p, d, f orbitals etc.

I realise that this is was a simplification and not necessarily an outright lie, and can understand why they did it this way. But it was the first time I realised I’d been deliberately taught something that was incomplete or inaccurate.