it is easy to do a lot of stuff "wrong" in super mario brothers. that's why it's so hard. the most notorious mistakes are made when confronting the king of the koopas himself, bowser.
you might think, "oh! a floating platform. i must jump on that and over bowser!" or "oh! a bunch of blocks! i must jump on them and then over bowser!" NO. don't jump over bowser. i mean you can jump over bowser, but DON'T.
pls, for the love of god, run under bowser.