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This is strange as I already had another flash movie on there which had not done this. The new one did use some code I got from the internet to create a visual effect - could this have been the root of the problem?
The lab has since updated their flash player and the problem is now fixed. I'm just wondering if this is something I need to worry about? I don't want this to happen to other people visiting my site, how can I make sure it doesn't happen?
My first guess is just that, an infinite loop or memory intensive process could have brought this about. The help files have a lot to say about optimizing memory - a Flash with lots of blends and transparencies can eat up memory very quickly. Sloppy coding can do it as well. For example, if you were to create a slide show that creates objects on the fly, and doesn't completely destroy unused/invisible objects, the Flash will run slower and slower until it freezes the browser. Again . . . sadly . . . from experience.
But in your case, since you said updating the Flash fixed it, it's most likely something being used by a later version that's not supported in older versions - for example, ActionScript 3.0 played on a player that only supports ActionScript 2.0. Ordinarily it just won't "work," but it's entirely possible it was trying to and encountered some method that wasn't supported.
This being the case, you might do a Flash version test before displaying the Flash. That is, with SWFObject or even home-grown Javascript, determine the Flash player version at the client, and if it's an old version display the update link instead of playing the SWF.
var FLASH_VER = getVersion().split(" ");
FLASH_VER = FLASH_VER[1].split(",");
if(FLASH_VER[0] < 8)
{
_root.gotoAndStop(20); //Display update flashplayer link for example...
}