A Wrench in someone plans

How do I properly subvert the Xanatos Gambit using this trope Spanner In the Works without going to Melodrama or Contrived Coincidence? I wanted to be that there is a lack foresight on the part of the planner while the person who the wrench destroys most the plans without knowing. Another idea in my head is that things do go as planned but not in the way they expected or at great and terrible cost. What can I do to throw a wrecnh at a The Chessmasters plan where go things go awry? how does planning work?

That...is a very nebulous description.

Sorry about that but how do I derail a Chessmaster's plans without having to rely on anyhting toio convient?

Study the plan. Might be an inside job, though. Find a weak link in the plan - an event that has many input lines and output lines, which is focal. That or a subprocess that is crucial to the final product. Could be something small. Hopefully, undetectable until it is too late to prevent the plan from falling apart like a house of cards. It would help if we knew a few things about this master plan, then we can take shots at it until you come to the point where the planner hadn't considered a potential setback that we toss your way.