I just did our upgrade to 2.0 (I'm a year behind, I know!) and it seems my custom template, even after upgraded with the new override code, is returning what appears to be content from other existing posts or articles elsewhere on the site.
The start date and other content appears to be different for some reason (that event is scheduled for February but I'm seeing January 2nd and October for some reason? Also, all the article content below the event summary does not belong to that event, or any event. In fact, this content you are seeing displays the same for all events created, new and existing that were migrated from 1.6.5.
Any light you can shed or questions you might have to help me get on the right track would be great. If I can't get it soon, though, I will probably just deactivate, remove and install 2.0 fresh and try again. But it seems to be working aside from this mystery content and formatting, so I'm not ready to go that route yet.