Voldemort: I need to get a prophecy from the ministry of magic, which is completely unguarded at night (except by a few pesky OotP members, but apparently they only guard sometimes, alone, and fall asleep on the watch), but only I or that boy-who-shall-die can retrieve it, and I can't go into the MoM because someone might see me, even though I am perfectly capable of apparating and using flu powder and there are no alarms to detect my presence, but I shall try to sneak in by possessing my snake, because that won't draw any attention, and then, when that doesn't work, instead of sneaking in with my minions (who are cozy with the ministry) and an invisibility cloak and taking the prophecy, I shall spend forever trying to lure that boy-who-shall-die-and-always-has-to-play-the-hero-and-make-a-huge-scene-and-has-OotP-connections-up-the-yin-yang to the MoM all the way from his school across the country so that he can get the prophecy for me.
Harry: I need to see if Sirius is alive. He might be in London being tortured at the MoM, and if he is, then I have to rush to London to save him. He gave me a mirror to contact him, but I'm going to forget about that and use flu powder to stick my head into the fire and shout from the basement of his 5-story house IN LONDON to see if he's home -- oh, crap, he's not answering, so why don't I hop out of the fire and really check the house, because then I'll be right there in London anyway... oh, wait, I can't do that, because then I would find out that he's home, never go to the MoM to save him, and then he wouldn't die trying to save me when I mess up trying to save him.