I just finished two rooms with hardwood flooring. I was told that the
flooring should go perpendicular to the joists. In one room I
remembered that. In the other I forgot.

The theory is that there's a tiny bit of flex between the joist that
will eventaully cause the flooring to sqeak and maybe bow ever so
slightly (but visibly).

I certainly don't plan to rip all the flooring up but I would like to
know just hw serious of a mistake did I make.....