Analysis / Didn't See That Coming

Problems tend to come in five varieties:
  1. Known Knowns: There are the known dangers, these you deal with directly: My opponent has access to laser blasters — I need Deflector Shields before I attack.
  2. Known Unknowns: These are the things strategists know they don't know, but they can prepare for the different possible outcomes: The enemy has his forces deployed elsewhere when you are Storming the Castle, so you need one plan for what to do if these forces do not return in time (to take full advantage), and you need another plan for what to do if these forces do return in time (to avoid getting trapped in a pincer). Someone who is especially good at this kind is probably Crazy-Prepared or a master of Xanatos Speed Chess.
  3. Unknown Unknowns: Then there are the unknown unknowns, or even unknowable unknowns. These cannot be prepared for, planned for, or in any way anticipated. They are the bane of all well laid plans. This can often be a Genre Shift, for example, and the character was in the dark because of the masquerade. The mage being surprised by the space alien, for instance, or The Mafia not expecting Psychic Powers. Acts of God and Outside Context Villains tend to fit in here, unless characters are aware that they are a Cosmic Plaything, and even then it's hard to know what you don't know, except that you don't know anything.
  4. Unknown Knowns: It happens. Sometimes the plotter knows a given person, event, or variable is present... but doesn't see how it could possibly impact their foolproof plans and proceeds to dismiss it or mistreat those "irrelevant" to his plot, and otherwise ignore the sword overhead while whittling at the rope holding it there. In Mystery Fiction, these people are the suspects and facts that are quickly discarded in favor of the high-profile suspects. However, add up all the background chatter and the motivations for why it was the butler becomes incredibly obvious. See The Dog Bites Back and Evil Cannot Comprehend Good
  5. False Assumptions: Even the most diabolical scheme can be doomed before it even starts if it's built on faulty information. That ridiculously circuitous plan to steal the MacGuffin from the safe deposit box by sneaking into the bank disguised as security guards? The only thing The Hero's grandfather hid there was some old family memorabilia; the MacGuffin is hidden somewhere else entirely (if it even exists).

Known Knowns, and Known Unknowns, are possible to plan around, and thus usually don't qualify for this trope.
http://tvtropes.org/pmwiki/pmwiki.php/Analysis/DidntSeeThatComing