It's a slippery slope.

Meaning

It's a dangerous course of action that could lead to undesirable consequences.

Examples

• John: Let's just bend the rules a little bit this time.
• Alice: I don't think that's a good idea, John. <b>It's a slippery slope</b>.
• John: But it would make things so much easier.
• Alice: I know, but once we start, it’s hard to stop. This is a serious issue; <b>It's a slippery slope</b>.