Ouch, sorry to hear that. It’s actually technically possible to review and reverse any changes made throughout your current sync history (which includes any changes within the last hour, as well as any changes not yet synced to all your devices), but we haven’t figured out how to put a good user experience on that yet so right now it involves working directly with your database transaction files.
If you ever need help recovering something like that, I recommend making a quick backup and then contacting our support humans who can walk you through the process of locating and reversing that change.
(As for potential ways of exposing this to the user: it might be weird to have Undo suddenly start undoing individual changes synced from other devices; from your device’s point of view it might make the most sense to have the entire sync operation show up as a single Undo, since those changes landed all at once from your device’s perspective. Of course, those changes were applied out of time—some of the synced changes may have been applied to your database before your local edits—so reversing that is somewhat complicated. What might be even cooler and more useful would be to show you a timeline listing each of your recent changes, letting you view your database at that point and selecting which changes you’d like to keep. That capability is present in the underlying data/sync model, but it would be a lot of work to bring that capability up to the user interface—work that would be interesting to do, but might not be as useful to the app’s core purpose as, say, letting you reorder tasks in Forecast.)