Poofing is resource-intensive on the server, so you should be careful how many mailboxes you update at one time. The larger the Calendar, the more time and resources it will take. (Please note that the POOF tool is not a supported tool. It is sample source that is provided as is without warranty.)
More from Stephen’s blog:
“OWA doesn't know anything about expanding recurring meetings and appointments. Instead, it relies on a task which runs in Exchange that expands the recurring items for it. Under certain scenarios, Exchange may not realize that a recurring item has changed and needs expansion again. Sometimes this is because of a bug, sometimes it's because of corrupted data. Sometimes a little of both.
“Anyway, the process for fixing this is called Poof. I think the name stems from the expression "Poof! Be gone!". When a Poof is performed on a calendar, Exchange deletes all of the cached expansions and performs them again.”