can of worms, it's a/like opening a

can of worms

A situation that, once started, is likely to become problematic or have a negative outcome. Getting involved in the minor border conflict has become a can of worms for the country, with no end to the military engagement in sight. You can try reformatting your computer, but once you open that can of worms, you'll probably be working on it for days.
See also: can, of, Worms
Farlex Dictionary of Idioms.

*can of worms

Fig. a very difficult issue or set of problems; an array of difficulties. (*Typically: be ~; Open ~.) This political scandal is a real can of worms. Let's not open that can of worms!
See also: can, of, Worms
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.

can of worms

A complex unexpected problem or unsolvable dilemma, as in Tackling the budget cuts is sure to open a can of worms. This expression alludes to a container of bait used for fishing, which when opened reveals an inextricable tangle of worms. [1920s]
See also: can, of, Worms
The American Heritage® Dictionary of Idioms by Christine Ammer.

can of worms

n. an intertwined set of problems; an array of difficulties. (Often with open.) When you brought that up, you opened a whole new can of worms.
See also: can, of, Worms
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions

can of worms

A complex or difficult problem.
See also: can, of, Worms
American Heritage® Dictionary of the English Language, Fifth Edition.

can of worms, it's a/like opening a

Introducing a complicated problem or unsolvable dilemma. The metaphor alludes to the live bait of fishermen. In a jar or other container, they form an inextricable tangle, wriggling and entwining themselves with one another. The term is American in origin, dating from the mid-twentieth century.
See also: can, like, of, opening
The Dictionary of Clichés by Christine Ammer
See also:
  • a can of worms
  • can of worms
  • get into a mess
  • addict
  • addict (one) to (something)
  • addict to
  • get itchy fingers
  • itchy fingers
  • get someone's back up
  • have back up