We released a new style of success and error messages with the calendar sidebar changes a few days ago. We've made them more apparent, which was distracting when there is a message for every action on the calendar. We've now removed several of the success messages for actions like rescheduling or deleting an appointment, you will still see error messages if something goes wrong. We've also made these messages stay on the screen for less time.
There was a bug with the new messages that would cause them to get stuck when they were displayed on a page opened in a new tab. This has been resolved.