The thin end of the wedge
Definition 1 of 1
Pro Tip 1/3Slippery 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 2/3Small 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 3/3Idiomatic 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.