Not greeting people over doorsteps and not turning back on your way from home. First is so ingrained in Poland, I thought it's good manners until someone mentioned it. The second is allegedly about bringing bad luck home with you, but IMO it's just common sense/punishing absent-mindedness away. Regardless of motivation, I consciously avoid doing either.
So you are not supposed to greet someone over a doorstep? How else do you do it?
greet = shake hands, sorry, to me it's practically a synonym. Doesn't matter if you go out to greet them or wait until they're in, just don't do it over the doorstep. No idea about the genesis of it as bad luck superstition, but it was either a fashionable influence from the outside that made more sense in context of parent culture or one of those old-Polish customs that MikoĊaj Rej would think archaic. 50:50 with those things.