When I set a global value through a script, FMP sometimes retains the previous value and will not let it be overwritten. This happens intermittently and unpredictably.
I have also encountereda problem where individual script steps suddenly stop functioning. Deleting the script step and replacing it with the identical step (and all the same parameters, if it requires parameters) remedies the problem (but for how long, I don't know).
In one script that I am working on right now, I tired to solve the first problem (global field not accepting a new value) by replacing the script set which set the value. In one instance, it worked. On the other, it won't -- no matter how many times I replace the step. The only difference between the two steps is that they target different global fields in the same table.
I am about to re-write everything to use variables instead though, in the past, I have found variables to be less reliable than globals. I don't know why but sometimes a global field just works where a variable does not. I have heard a lot of other developers remark on this, but no plausible explanation or rule for when to use variables that I can rely on, so I have been using only globals to be safe.
Needless to say, these two apparent bugs in FileMaker 12 are extremely serious. For several months, I have been working on solutions for two different clients which could be rendered useless if globals or variables do not work reliably. This effectively makes FMP 12 unreliable as a development platform until these problems can be addressed.