I've been going through the XEvents in Denali CTP1 in a fairly rigorous amount of detail, looking for what's changed and specifically for breaking changes. Running all my old event sessions… found one.
One (well OK, quite a few) of my "for illustration" XEvent Sessions have a predicate that looks at the error number for the event sqlserver.error_reported. In SQL Server 2008/R2 XEvents, this predicate would be something like "where error = 999", but Denali the event field for "error" in this event is "replaced" by one for "error_number".
This is a small change but a breaking change, meaning that all event sessions that rely on the event field "error" would break. There are some additional event fields for this event in Denali (so there's probably a good reason for the change), but since I don't think I'm the *only* person with event sessions referencing this event and field, I thought this would be useful information to share. The system_health session uses this event field too, and it (naturally) had to be updated.