can of worms

Short definition from Wiktionary

Noun

1. 1
   A complex, troublesome situation arising when a decision or action produces considerable subsequent problems.
2. 2
   A troublesome situation; an issue whose resolution is difficult or contentious but not necessarily complex.
3. 3
   Any difficult or disgusting task, activity, topic of discussion or subject matter, which would be ideally avoided if at all possible.
4. 4
   Used other than figuratively or idiomatically: see can, worm.