Due to the way Minecraft is coded, saving of the world and player files is periodic (occurring every 10 minutes or so). When your server crashes or freezes and the server starts back up, it must reload from the last save, so the world may be rolled back as much as 10 minutes.
If you see a rollback, it will have been from a freeze or crash. While these aren't exceptionally uncommon with Vanilla, they are more often associated with faulty/outdated plugins. We recommend frequently checking for updates to your plugins at http://dev.bukkit.org/server-mods/
, if they're made for CraftBukkit. In addition, after a crash, you will see output with more information on the "Events log" page, and this should hint at changes you might need to make to the server in order to increase its stability. In cases where the server doesn't crash (and instead freezes), watch for runtime errors in the streaming console output for your server on the "Server control" page.
You will not see a rollback from the normal daily restart or a restart made through the "Server control" page in your control panel, because our system tells Minecraft to write changes to disk and to gracefully stop as part of that process, resulting in all of the data being saved.