I think footguns are something thats 100% unexpected. I would argue that a user of a database that sees any kind of reasonable size ought to be very aware of how things work, because to be blunt (to use same language as GP), if you are caught off by this, chances are that you are not qualified to tackle the thing you're doing. It is scary how many in high positions have no clue about indexes, and either are unaware, or have gross misconceptions about how things work