Three Things

"Three sisters2" by USGS Photograph by Lyn Topinka. - http://vulcan.wr.usgs.gov/Volcanoes/Sisters/Images/framework.html. Licensed under Public Domain via Wikimedia Commons - https://commons.wikimedia.org/wiki/File:Three_sisters2.jpg#/media/File:Three_sisters2.jpg

A developer must know three things to properly support an application:

Expectation: What users believe the code does.

Intention: What the original developer intended the code to do.

Reality: What the code truly does.