The problem is compounded by the fact that there are actually two different issues affecting the same feature.
The first bug, and the more serious of the two is that calendar events that are created by users or the ones they have accepted disappear from the iPhone for no apparent reason. This means that users relying on ActiveSync could end up missing meetings, conference calls etc. The weird thing about this is that these events are still visible in Outlook or via the webmail interface.
Users are also reporting issues while accepting or declining an invitation. Users are reporting that they get an error message informing them that your message cannot be sent. While this bug might not be as serious as the first one, this could prove detrimental to people who work in fragmented groups and use this feature to track each others schedules and event plans for the day.
source – iphonehacks.com