[Fern]

Open the Reward tray from event tools, pick any reward, then click a person and give them their reward.
  
Then press the delete button in your keyboard to hide the HUD, click a different person ONCE while that's being done, and then watch it suddenly give the reward to the previous person you gave it to again (without hitting Apply Reward, only clicking someone to select before doing that.)
 
This doesn't seem intentional (specially because sometimes you can't click someone if they're under the Event Tools tray, so you need to hide the HUD.) so I'm putting it here!

[Neus]

I've added a small safety for this to make sure you don't accidentally give a player the same rewards twice in a row.