I already handled this error when any product is saved using magento event observer but saving any cms page from admin i still receiving this error. Any help would be appreciated.
The main reason this happens is due to inclusion of Widgets in the page. If the page contained a widget it is likely that the parameters of the widget were either changed or it was feasible that they may have changed, so cache is invalidated.
As an aside, an invalidated cache does not necessarily mean that the cache is not operating. It means that the cache is now dirty and requires a refresh. Please do not mistake this as a systemic issue. It's merely the admin giving you a notification that there are potential updates that aren't visible to the customer.