Must not apply this update until you have updated your FileMaker Server installation?
Regarding security update:
Where it is specified:
Alert your clients (FileMaker Pro and FileMaker Go users) that they must not apply this update until you have updated your FileMaker Server installation.
Why is this the case that they "must not" upgrade? The summary page says:
To me "May not" and "Must" are very different. On a test machine, the upgrade works fine and connects with both 12 and 13 (un-upgraded) servers without a problem. What are the possible side effects or implications if we do upgrade some users before we upgrade the servers?