Open a can of worms.

Here we see a pair of fish having a discussion at the bottom of the sea. 

Fish love to eat worms, and one of them, named Frank, is tempted to open the worms with a can opener. He appears to have missed the fishing line attached to the can. 

The other, smarter fish, is warning Frank that opening the can of worms could mean a lot of trouble. Frank should listen to his friend's warning, unless he wants to be somebody’s dinner! 

The idiomatic meaning of 'open a can of worms' is to create a situation which leads to trouble. Let's take a look at this idiom in a conversation: 

Hey, Simon. Is Janine invited to your sister's wedding?
Sure. Why not?
Well ... Rhys is going to be there too. 
So?
Isn't that going to open a can of worms?
What do you mean?
They haven't spoken to each other since they broke up!
Oh, you're right! And they often fight.