Through the browsers, there are at least four different ways to implement Flash: IE, Firefox, Chrome, Maxthon ... Their updates are not synchrone. (Add systems different from Windows)
Each Flash update is likely to change the game's behaviour on the respective computers, regardless of WGT programming.
Apparently there has been an update of Flash in Chrome browsers leading to some problems - see different threads here for possible solutions.
It may be a good general action to disable automatic updates in all versions and let the local admin decide about it.
Edit: Flash 12 has been released for a number of browsers - good luck to all!