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.
Some environments such as Windows and PHP running in safe_mode are unable to use the simultaneous edits capability distributed with PmWiki. See Cookbook:SimultaneousEdits for a solution for these environments.