Daylight Saving Alert
If you run a site in a UTC timezone that uses GMT in Winter and observes DST (Daylight Saving Time) during the Summer (e.g. London/Europe uses BST (British Summer Time) in the Summer) and you are creating a recurring event series that spans the switch between GMT & BST time-zones (or vice-versa), then an alert should appear warning the user that it's not possible, and that they should create two event series that split the event either side of the timezone change. Otherwise, event in the series after the timezone change get incorrectly formatted.
Jim Houghton commented
I have an even better idea. Fix the plugin so that this doesn't happen. I've had several conversations with the developers but the theme-conflict persists. The way to fix it for the theme I'm using is to comment out all 10 instances of "date_default_timezone_set" in FTP. It's a pain in the butt and has to be redone after every theme update.
This is obviously not a problem with every theme or there would be an unholy number of people suggesting a fix.