Originally Posted by rm -rf

You can easily mess up the settings enough that when you shift to the biggest cog, your derailleur's pulley cage catches a spoke. Then the derailleur is mangled, ripped off the frame, and your rear wheel is locked up. Hopefully, you don't crash.

The Park Tool website shows how to start at the beginning, setting the limits correctly, and getting the shifts to work perfectly. It's not hard to do, and it's easy to understand once you've done it the first time.

It's worth knowing so that, in the future, when the shifting starts to be a little off, you can fix it in a few minutes.