We all have heard that the reason the Jedi could not detect Palpatine or Anakin's Dark Side tendencies for so long was because the Dark Side clouded their sight. But is this the only reason? I believe another factor was at play. No one, not even the Jedi, can control a human emotion that is activated when a crisis occurs. A crisis such as the fall of the Republic and the death of the Jedi Order, an impending doom that many were seeing before their eyes. This emotion is denial. When something as unbelievable happens, such as the impending fall of the Republic and the fast approaching end of the Jedi, denile is the most common emotion. To think that the Sith threat, something thought vanquished years ago had returned, was an unthinkable thought. To imagine yourself in that position would be horrible, and your mind would try and deny it, even though you truly know that it what is happening is real. This probably was yet another boundary the Jedi had to cross when trying to locate the Sith, along with the Clone Wars and the enshrouding mist of the Dark Side. This especially applies to the case of Anakin. It would be, and will become, one of the most horrible travesties of the Jedi if the Chosen One was to turn on them. This would especially be hard to believe for Obi-Wan. He tried to sway Anakin back to the light throughout their entire duel on Mustafar, although Anakin had already been firmly set on the path of darkness, and only through the near-death of his son would he finally be redeemed. After years in exile, Obi-Wan then accepted Anakin's fate, and wanted simply for him to be destroyed. These are all normal emotions that we feel, and it is amusing to me that the keepers of peace and order throughout the galaxy are suceptible to these mind-melting feelings of dire crisis just like the common, and very real person.