can of worms

a can of worms (a complex unexpected problem or unsolvable dilemma, a source of unforeseen trouble; a very complicated situation that causes a lot of problems when you start to deal with it) — ≈ сам чёрт ногу сломит (о запутанном или неразрешимом вопросе); сплошная неразбериха, нервотрепка

open (up) a can of worms
(If you open a can of worms, you do something that will cause a lot of problems and is, on balance, probably going to cause more trouble than it's worth; you reveal a situation that is bound to lead to trouble; instigate trouble) — сделать только хуже; навлечь кучу неприятностей;

≈ разворошить улей;
≈ хлопот не оберешься; неприятностей не оберешься

Example 1: I just don't know what to do — every solution I can think of would just open up a whole new can of worms. (LDOCE)

Example 2: Tackling the budget cuts is sure to open a can of worms. — Заняться проблемой сокращения бюджета — это огрести кучу неприятностей.

Example 3: The company opened up a can of worms when they decided to talk about the new contract.

If there’s a topic you don’t want people to talk about, you could call it a can of worms. (опасная, больная, неприятная для кого-л. тема для разговора)

Example: Let’s not talk about that. Don’t go opening a can of worms.

see also
[snowball effect]
[Pandora's box]
больной вопрос