Testing is critical. If you use Netscape, you probably cannot see the Flash-based message above. With IE or on a local hard drive with Netscape, the file will look just fine. As Web plug-in technologies go, Flash is pretty dependable and consistent. That does not mean that it always works as planned.
Software has bugs and all Web multimedia carries with it a certain percentage of failure.
Throw in human error and you have the potential for a Web site disaster. Like the job applicant I interviewed who had a Flash portfolio that did not show in Netscape. It was not a good way to make a first impression. But do use common sense. Test your work on every platform, browser, and plug-in combination you can get your hands on.