What a brilliant topic you have brought up here. Thank You. It is one of my personal favourites.
My families were all very superstitious, and all of the ones mentioned above were included along with tonnes more.
They said to go against the superstition would bring bad luck.
It never occured to me to ask what they meant by bad luck.
I just went along with the superstition because I didn't want bad luck, whatever it meant, it didn't sound nice to me, and if it made them afraid, then I accepted it was something to be afraid of, too.
I don't think they felt it would upset or anger any Gods or Goddesses, but I can't be sure.
All I know is that I still cannot go against the superstition, whatever it is, because there is real fear for me in them, even though I've no idea what will actually happen if I break them.
I broke a mirror a few weeks back, and because I forgot the way I'd been taught to deal with it to lessen the bad luck, I just boxed up the bits and took them to the local dump without ceremony or anything else.
A few days later my car broke down costing me £800.00 to fix, the breakdown truck recovering me broke down, and when my MOT came up it failed on the 2 front tyres.
Powerful stuff, then, this superstition.