I believe this is a bug, though I have no idea how to find out why its doing this - perhaps if someone can tell me how I can troubleshoot it, it will help.
I cant seem to drag and drop events onto the last 2-3 hours of the day. The problem does not seem to be consistent.
I created a quick screencast of the problem which is here http://www.youtube.com/watch?v=oGCZ42yaK1w - its a short 45 second video and you will see me trying to move the event and failing.
On top of that, you will also see on the video that when moving an event it doesn't seem to snap to the slots properly and trying to move an appointment to 9.30 will look like I am dropping it on 9pm.
Fixed
User interface
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It's likely this issue predates Contrib.social: some issue and comment data are missing.