title_body: What happens if you fail to clean a steam wand after frothing milk?
upvoted_answer: Word on the street is that:
  
  
  The milk residue will get "baked on" by the heat of the steam wand, making it even more difficult to clean than if you'd just done it when the milk was still wet.
  Any milk left on the tip of the wand will get sucked up into the machine's water reservoirs by changes in air pressure, where it will stay and become rancid and ruin everybody's drinks.
  
  
  This is according to Joel's biweekly beratings on the topic. We might ask him to cite his sources.