Now that there is no longer a global field type in FileMaker 7, I find that I often forget to go to the storage tab and click the Use global storage check box. Then later I find out the hard way. Something doesn't work and after all sorts of investigations I figure out that my global field isn't really a global. I feel pretty silly and have wasted valuable time.
Something that helps me avoid this is naming my global fields so that they all group together. In FileMaker 6, I was happily putting an underscore in front of each of my global fields like this: _InvoiceNum. That conveniently sorted my globals to the bottom of the alphabetical sort order where my clients wouldn't stumble over them very often. In FileMaker 7, the underscore sorts to the top, so I've chosen to put a zg in front of the field name like this: zg_InvoiceNum. Not pretty, but it accomplishes my purpose.
I now can periodically just go to the bottom of my table field lists as I am developing and scan through the Options column of my globals to make sure they are all set as globals. Such a scan would be a great checklist item before turning over a solution to your client. I put it on my checklist.