A Single Slash /

Just spent an hour trying to figure out what was going wrong in my routing. It was a forward slash. A forward slash.
Before:

<Route exact path="stacks/:id" component={props => <Quiz {...props} />} />

After:

<Route exact path="/stacks/:id" component={props => <Quiz {...props} />} />

See the difference?

What tiny mistakes have slowed you way down?