Related:

• If you perceive that there are four possible ways in which a procedure can go wrong, and circumvent these, then a fifth way will promptly develop.
  -- Murphy's Law Corollary 4...
• If you perceive that there are four possible ways in which a procedure can go wrong, and circumvent these, then a fifth way will promptly develop.
• If there is a possibility of several things going wrong, the one that will cause the most damage will be the one to go wrong.
  If you perceive that there are four possible ways in which a procedure can go wrong, and circumvent these, then a fifth way will promptly develop....
• If Murphy's Law can go wrong, it will.
• Murphy's Third Law: Anything that can go wrong will go wrong, and at the worst possible time.
• Anything that can go wrong will go wrong, and at the worst possible time. -- Murphy's Third Law
• Murphy's Fifth Law: If anything just cannot go wrong, it will anyway.