I've experienced the sync break of GTW and Marketo thing and only discovered it when the campaigns built around GTW and Marketo appear to stop working. However, this has not happened for webinars that are past.
For us, because this sync is attached to an ongoing campaign that has a start/end date and time, it's crucial that users be notified immediately when this sync breaks - one would surmise that an email to the users login email to be a sufficient form of notification. Then, in addition to that, Marketo has a built in "notifications" function at the top left of a users instance (after logging in), that would appear to be the most reasonable place to put such a secondary notification.
actually we have had several registration errors that never get passed over to GTW so the people never get login info or the GTW reminders. looks really bad. and we haven't ever solved it.
I create a campaign now for all my webinars to send me an alert when this happens. I then go in and see if I can tell why (maybe email is bad or something) and if I have good info I manually register that person again.
here is an example
any my alert is just a text email with the name, company and email of the person who didn't synch over.
Hope this helps!