"To go pear shaped is an expression used to indicate that a scheme has not been perfectly executed.
The phrase seems to have originated in British English in the late 1940s or early 1950s. I have come
across several suggested origins, but the best, for me, is related to training aircraft pilots. At some stage they are encouraged to try to fly loops - very difficult to make perfectly circular; often the trainee
pilot's loops would go pear shaped."