When you use a candle on a cake that already has a candle on it, the candle will duplicate and the candle doesn't seem to be taken out of your inventory or hotbar. 

How to recreate this glitch:

1. Have a candle in your hotbar and a candle on a cake in front of you.
2. Place the candle in your hotbar onto the cake. There should now be three candles, one in your hotbar, one on the cake, and one on the ground.

This happened in the newest snapshot (added Dripstones).