Yet another beautiful reddit example of what can go wrong internally within your braking system.

In this particular case, a sticking caliper specifically wore down one side of the rotor pictured here.This typically happens once all the friction material on your rotor has been worn off, allowing the bare metal backing to grind in to the naked rotor. Fortunately, the chances of your caliper sticking is very minimal. However, in the off chance your caliper does stick, it can be very hard to fix. According to doityourself.com, the process to fix your sticking caliper is very extensive and requires a mass amount of handiwork.

Want more information on how to fix a clinging caliper? Read this article