This type of sideways burn is called “canoeing.” It’s not uncommon and can be caused by anything from poor rolling, to the wrapper leaf itself, to breezy conditions assuming you’re smoking outdoors.
Rotating the cigar while your smoking it can help in some instances, but if you’ve got at least an inch or more of unburned leaf I suggest you try the following repair technique.
- Let the cigar go out completely.
- Burn off the excess leaf with your lighter, place the cigar back in the ashtray, and let it burn itself out again.
- Clip the cigar about 1/2 inch at what remains of the foot, which will give you fresh tobacco to start with again.
- Toast and light the cigar again making sure that you get a good even light across the foot. Blow on it to be sure all the tobacco is glowing.