PmWiki has support for handling the case where multiple authors attempt to edit the same page nearly simultaneously. Here's the basic scenario for systems where simultaneous edits are not handled:
PmWiki's simultaneous edit feature detects when this occurs, and instead of saving Alice's edits PmWiki presents Alice with a message that someone else changed the page while she was editing it. Furthermore, Bob's changes are merged into Alice's copy of the page, with any conflicts highlighted by <<<<<<< and >>>>>>>. Alice can then fix things as appropriate and save the updated page, or, if Alice is lazy, she can just hit "save" a second time and leave it to someone else to fix.
The simultaneous edits feature is also invoked whenever someone requests a page preview; thus if a page changes while previewing a page the author gets notification and can see the merged results.
When your webserver runs in safe mode, it will not be able to run some routines that are needed for correct handling of simultanious edit, as descibed above. In stead of conflicts highlighted you will be presented with an empty page in edit mode. When you encounter this problem, go to Cookbook:SimuleditWithoutPopen for the solution.