The number one thing you break on any sort of quadcopter or drone is the propellers. With this pack of Tali H500 propeller blades you will make sure you have extra for when you inevitably break a couple. These blades are self tightening so you don't have to worry about them coming undone when you are up in the air.
If you are thinking about getting a few of these you might as well check out an extra battery as well. Here is the whole Tali H500 if you haven't gotten that far yet!