The classifier got one wrong — how do I undo and how is it remembered?

On the From your calendar page during your evening check-in, every event has two buttons next to it, and they do different things:

1. Undo removes the log row that was auto-saved. That is all it does. The same event will not be re-added, but a future event with the same title may still be suggested again.
2. Never suggest again removes the log row and remembers that you opted out of this title. The next time an event with the same name shows up, it is skipped instead of saved.

So for a one-off mistake, tap Undo. For something like "Dentist appointment" that you never want logged, tap Never suggest again and you will not be bothered with it on future syncs.

What if I change my mind?

Open the menu (gear icon), tap Manage Items, and find the Calendar opt-outs card. Every title you have opted out of appears there. Tap the bin icon next to a title to forget that opt-out. The next time a matching event shows up in your calendar it will be classified fresh and may be auto-saved again.

Will the same event get logged twice over multiple days?

No. The mapper remembers each calendar event, so even as a future event moves closer in the window (from 6 days away to 5, to 4, and so on) it never gets re-added. Your earlier decision on that event stays.