The thin end of the wedge

Pro Tip 1/3

Slippery Slope

Suggests a step that leads to a series of negative events, often irreversible.

Allowing students to use phones in exams is the thin end of the wedge, next they'll want open book tests.

Pro Tip 2/3

Small Cause

A minor or initial action that causes a significant impact over time.

Skipping one class may seem innocent, but it's the thin end of the wedge for poor academic performance.

Pro Tip 3/3

Idiomatic Warning

Used as a cautionary statement about the start of a potential problem.

This minor leak is the thin end of the wedge for future plumbing disasters in the building.