Unfortunately, just because you shouldn't NEED to do something doesn't mean you won't! (I speak from unfortunate experience here!)
I should have made it clear in my previous post that Cut and Paste will work to move items, including global variables, from one sheet to another. Copy and Paste is where the danger lurks.
If you copy and paste, rather than cut and paste global variables, and then make the mistake of deleting the original globals, you'll essentially trash any events that used those original global variables.
Many people tend to avoid Cut and Paste, considering it more dangerous than Copy and Paste followed by delete, but in this case the reverse is true.
I've reported as a bug that you can delete global variables without any warning that the conditions, actions and expressions that reference it will also be deleted. EDIT: Ashley says this will be fixed in the next build.