Kalecgos is one of them, that boss should just be wiped off the map. He's just that annoying. But baring that, there are these lovely little hedges placed around the instance. Don't go on those. If you do, you might just make your raid leader and Warlocks scream at you.
If you get on one of these hedges and step the right way, you'll find yourself stuck behind them (click the image above for a high resolution version), unable to get out. The only way to get out is for you to hearth, for a Mage portal to be placed behind the hedge, or for a Warlock to summon you out. All the while you've taken up a good couple minutes of raid time.
Needless to say, I've found myself in this trap before. Sometimes these hedges can be particularly deceiving since there'll be a space behind them that it looks like you can run around in and probably find a way out. However don't be fooled – 99% of the time there isn't.
Maybe this is something the Blizzard modeling team can take up. Moving the hedges back just a few feet so they're flush against the wall would do the trick. But until this annoying little environment "feature" is changed, watch your step in the Sunwell Plateau!