[sciolistse]

Yeah on original NES/SNES controllers the rocker physically prevented this. I was mostly referring to some third party controllers like say the 'hitbox' which go through an input cleaner before sending it to the system.

The way emulators handle L+R depends a bit.. For SMB1 specifically you're allowed to use an up to date version of Nestopia, Bizhawk, FCEUX or OpenEMU, all of which by default give neutral inputs if you press L+R (I don't recall if OpenEMU works this way but I believe it does.) There are other emulators like older versions of Bizhawk which gave priority to your latest input, or older versions of Mesen which gave priority to left and down. I'm sure other methods of handling L+R exist as well.

Which method is most correct is debateable, and sometimes contentious! :)