can of worms

(idiomatic) A complex, troublesome situation arising when a decision or action produces considerable subsequent problems.
(idiomatic) A troublesome situation; an issue whose resolution is difficult or contentious but not necessarily complex.
(idiomatic) Any difficult or disgusting task, activity, topic of discussion or subject matter, which would be ideally avoided if at all possible.
Used other than figuratively or idiomatically: see can, worm.