That's a can of worms.

Meaning

That's a complex and potentially troublesome issue.

Examples

• John: 'We should review the legacy code.'
• Alice: '<b>That's a can of worms.</b> It's incredibly complex and poorly documented.'
• John: '<b>That's a can of worms.</b> It involves multiple international partners and regulations.'
• Alice: 'Let's address the supply chain issues.'