At the risk of offending the fine members of these professions... hair stylists and makeup artists tend to cause the most delays.
Some slippage is understandable; often the hair stylist is asked to do the hair of the bride and the bridesmaids, many of whom he or she has never seen, much less worked with - plus some bridesmaids think your wedding is a great time to try out a new and time-consuming style. If your hair schedule slips and your makeup schedule slips... you can easily find yourself half an hour or an hour behind, especially if the bride is the last person to have her hair and makeup done.
Our advice? When you meet with your stylist and/or makeup artist, stress the importance of your schedule, and make sure they know exactly how much time they have. Then, have someone in your group periodically remind them where they stand so they'll stay on track on your wedding day. (If you're the bride, appoint someone else; the last role you want to adopt on your wedding day is the role of the heavy.)
