I think during the game, the wrong thing for a DM to do is think like a DM or even worse a PC. They have to think like an NPC, which is a slight variation, because an NPCs job isn't to be the centre of attention, but as a backdrop and second to the interests/motivations of the PCs; to assist in the story, nothing more.
Sometimes, a DM feels their job is to prod the characters or story along a certain path, which is fine for long term planning outside actual playing time...thinking about how the events in a general sort of way "should" or "can" go. But the game has a flow all to itself, and the DM can only play the characters that are in play, and should not try to use a heavy hand to micro-manage the events, even if the players sometimes make "mistakes" in terms of where you want the story to go...accept them and respect the players choices. Their choices then becomes the story...analyze the situation and the consequences after the game and then go from there.
The story is not yours to control fellow DMs, be the npc when it's time to be an npc, but nothing more...but that's just MHO.
dren