I've been working with using Pixel Bender shaders in Flash and have encountered plugin / browser crashes when using shaders on large images, but only on certain machines.
I've been unable to reproduce the crash on any machine I have physical access to, but was able to determine that it was caused by executing a shader on an image over 1028x1028 in size. I created a test Flash file (http://www.saltgames.com/?page_id=283) that runs a simple shader on gradually increasing image sizes. On most machines it will run fine, but on a few the Flash plugin (and possibly browser too, depending on the broswer) will lock up once it tries to render the 1500x1500 sized image.
One user reported that although it crashed when using Firefox or Chrome, in Internet Explorer on the same machine it was fine. I haven't been able to see a common feature in the set-ups that suffer this crash.
I had approximately 50 people test a Flash game that featured shaders operating on large images, and 4 reported a crash. Changing it to only run shaders on smaller image sizes seems to have fixed the crashes.