Suppose you want to use Flash on your website. I think this is a bad idea, but I accept that some are going to ignore my wise advice and do it anyhow. If you do this and you’re not careful, you can make it absolutely impossible for some people to see your show.
Consider for example thesixtyone, a nice sort-of-game-structured music site that I use for background when I’m working on something that’s not particularly taxing. It’s lively and well-designed and dynamic and an example of intelligent and graceful Ajax. Only it wouldn’t work for me when I first visited it.
The reason is that it uses Flash to play its music. On both Camino and Safari, like many other people I use a Flash blocker; this disables Flash by default and shows me some sort of graphic indicating that there’s Flash there if I want to click on it. The number of irritating squirmy lame-ass ads that I never have to look at is remarkable; and when I hit YouTube or something like that, I click and watch.
On thesixtyone, they’d hidden their Flash player, made it invisible somehow, so there was nowhere to click to make the music go. Don’t do that!