I always thought having to shoot 5 missiles in a row to open the red doors was dumb.
If you're an average player, you would try shooting it with a beam. Nothing happens. "Lemme try shooting a missile. Nothing happens. Try one more time. Still nothing. Okay, I guess I just can't open this door yet." Who would think to keep trying this thing that clearly isn't working?
I never got stuck but I can understand why someone might. :P
Back in the day there were instruction books that held such knowledge. Although if one has never played a Metroid game before and didn't have an instruction booklet I could see the confusion.
Edit: Actually the opening idle screen does show how to use them to open doors.