The ACF design doesn’t have any design for synchronization. What happens if two users submit forms at the same time? Either one or the other should complete in it’s entirety before the other begins. For simple file writes, it shouldn’t be a problem. However, some ACF actions are now accessing databases, and other slow functions with multiple steps.

Not sure if this is an actual problem or not, but I’m assuming it is. I’m not aware of how mini_httpd (the default www server for ACF) manages CGI scripts. So, this will require some testing.

Possible solutions:

• Ignore the problem
• Add a synchronization object that all ACF actions will grab before making changes
• Rewrite ACF with a daemon that implements (and synchronizes) all actions

(from redmine: issue id 635, created on 2011-05-10, closed on 2016-05-31)