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For the most part, you don't have to worry. OmniFocus takes the last change made to any given item and discards any others. Note that this is done on an action-by-action basis, not the individual bits of data in an action, so if you change the due date of an action on one machine, and the context on another, then sync, one of those changes will be lost (and there won't be any warning that this has happened). A more likely failure is to mark a repeating action or project as completed on two different devices, which will cause duplicates to be made of the repeating item, because the replication happens when something is marked completed, and the software isn't clever enough to recognize that something has been marked completed on two different devices and merge the two chains together (could be rather difficult if you've then made changes to the newly created items, so it isn't surprising to me that they haven't tackled that issue).

Best practice is to do a sync before starting to make changes on a device, and after you've made some changes, too. Normally, OmniFocus will do a sync every hour when it is running (and in the foreground in the case of iOS versions), but if a change is made, it will attempt a sync 1 minute after a change is made, then return to the 1 hour cycle. So, if OmniFocus is left running after you make changes, it should probably do that post-change sync without you needing to do anything. The only time you can have possible sync issues is if you make changes on two or more devices without syncing in between, and even then it isn't an issue if you didn't change the same items. Doing syncs in this way optimizes the transaction graph and makes it likely that OmniFocus will compact your database more frequently and effectively. That reduces the time it takes to sync, and makes it easier to sync first and ask questions later :-)

The one thing OmniFocus sync can't do at all is merge two different databases. If you've got the Mac version, have been using it for a while, and you decide to buy the iPad version or the iPhone version, they are so much fun to use that it is tempting to enter a bunch of data before you set up sync, but you'll be an unhappy camper when you try to sync the first time, because it will want to discard one of the two databases. It isn't impossible to recover from this, but it is a headache best avoided!

In several years of use and many thousands of actions synced between Macs, iPads, and iPod, I don't recall seeing a single sync mishap that wasn't ultimately my fault (edited the same item or completed a repeating item on two different clients without sync in between).