As an experienced chess player, there are some situations in which people who know the rules still might get tripped up on. For example, consider the situation:

    K R r
    _ _ _
    k _ _

Can `k` move to the right? Someone who isn't familiar with this situation might think so, because `R` is pinned to `K` by `r`, but it turns out that even though `R` can't move to the square right of `k`, `R` can still give check to it, so the move would be illegal.

Then you also get people who don't know about rules regarding castling through check, etc.

Basically what I'm getting at is, there are moves that even people who casually play chess might misunderstand, so OP was implying that they were not one of these people.