Such as jagged tops or sinkholes. These are easy to fix and I would suggest a couple of options.
For sinkholes if you have some extra wax you could always pour some cooled wax into it. About when it is at the point of gel. Not too hot as it would start to melt the candle parts that are set/done.
The second thing you could do you would also use for jagged tops. Use a heat gun! Warm up the top later to smooth it out if it is jagged OR if you are fixing sink holes it will melt the top layer of candle enough to drop inside and fill it's own hole back out.
NOTE: Holes generally only occur with taller candles. As the candle cools it shrinks. Which is why you should poor wax too hot or too cold. More of the gel stage unless you heat up your glass. Smaller candles like votives and those up to about 3" tall are typically perfectly fine. The taller the candle the higher the chance this will happen.
Here are some videos to help with illustration.