Testing using different displays
I do a lot of web testing, so it's easy for me to remember to test in multiple browsers. I know each browser renders content in it's own way. I also often remember to test using different screen sizes, because I know sometimes content gets truncated or smashed together on lower resolutions. However, sometimes you might even want to try testing with different displays. Sometimes you'll notice contrast differences between brands or types (CRT, LCD) of monitors, or even devices (think mobile applications). Depending on what you're testing, this distinction might be important. For example, if your developing the UI for an ATM system, knowing how you render on CRT machines vs. LCD machines might really be important.