I am idly curious as to whether there is a word for someone who is prone to scheming, but not terribly good at it.
The phrase hoisted on his own petard conveys the concept
to suffer harm from a plan by which you had intended to harm someone else
The phrase is used by Shakespeare in Hamlet
For ’tis the sport to have the engineer
Hoist with his own petard
An engineer is a builder of military equipment, and a petard is a type of bomb to blow a hole in a wall. In effect,
it's amusing to see the would be bomb-builder blow himself up.
The Warner Bros. cartoon character, Wile E.Coyote, is an embodiment of the concept, scheming to destroy his nemesis, The Roadrunner, but constantly being defeated (and injured in grandiose fashion) by his own conniving.