This is a bug of such astounding enormity it beggars belief, and compromises pretty well every solution out there.
Turns out that in FileMaker 16 in Windows, users can now close windows while scripts are running and while Set User Abort is [Off].
Part of our system previews Invoices, then immediately saves them as pdfs and e-mails them direct to customers. Previously everything worked absolutely fine, and we never had problems. Now users can potentially close the preview window by clicking on the [X], and the user is left in a Pause situation without knowing how to get out of it. Maybe they try pressing enter or return a couple of times to see if that helps. Doing that then merrily saves all the records (wherever the user happens to end up) as a pdf and sends them direct to our customers. Could be customer names and addresses, could be a list of invoices, could be any screen that is open in the database. So our system has been churning out all sorts of random things - potentially confidential and highly sensitive - which could easily lead to complaints, quite conceivably legal action.
I sit here, and really can't believe it still. I've spent over two weeks sifting through the system in forensic detail trying to find the source of the issue, and to be honest I'm very lucky to have found it before we sent out something in error that led to really serious repercussions.
I didn't spot it on Mac during development as in FM16 Mac this bug isn't present. Production environment is Windows and that's where the problem has been happening.
Allow User Abort [Off] behaviour has been the same for as long as I can remember. This must have been something that's just slipped through the net, what with all the new windowing changes in 16. Of course when we are scripting now we can leave the Close [X] off by setting the new window options, but exising solutions MUST run as they always have without a significant change in behaviour like this that can totally compromise solutions.
This is a bug that could be wrecking data for everyone and needs addressing urgently.