It's a slippery slope.

Meaning

A seemingly small action could lead to serious negative consequences.

Examples

• John: Maybe we should just skip this step.
• Alice: I wouldn't recommend it. <b>It's a slippery slope</b>; skipping this could cause significant problems later.
• John: I understand your concern, but it seems minor.
• Alice: It might seem minor now, but I assure you, it's <b>a slippery slope</b>. We should be more cautious.