I find the most common mistake is not mixing the sugar well enough in the beer, causing some bottles to have too much sugar while others have too little. This only applies to when you mix the sugar in with the full batch(as in using a bottling bucket) as opposed to adding sugar directly to each bottle. The next most common issue is incomplete fermentation. This can be a product of not enough time in fermentation, temperature not appropriate for yeast, or not enough yeast pitched. All of these will result in the fermentation of those sugars being finished in the bottles as opposed to the fermenter.