A very small example is on OneRx. This is one of my favorite stories because it shows the importance of testing and what a difference small things can make. OneRx is an app that allows users to price compare medications at nearby pharmacies. 

With this feature we were allowing people to save their medications. At first, we didn't have the green check mark. It was just the slider. The slider would be gray and the "Add Medication" would be regular font and then you'd tap it and the slider would move and turn green and the font would turn bold (the messaging would also chance to say "Delete Medication"). Except when we tested it people weren't totally confident that they had added the medication. They all thought they did, but they weren't too sure. 

Then we added a gray plus that would turn into a green checkmark when selected and immediately every tester was completely confident they has completed the task, crediting the green check mark.