Tunnelling
When a candle burns a narrow hole down the centre and leaves a wall of unmelted wax at the sides.
Tunnelling usually starts with a short first burn: the wax 'remembers' the small melt pool. To prevent it, burn the candle until the melt reaches the edges each time (about an hour per 2.5 cm of width). To rescue a tunnelled candle, wrap foil around the rim with an opening in the middle and burn it for an hour so the edges melt down.