It is vital to having a healthy production that you take the necessary amount of time and effort to determine all major conflicting situations. If your list is not a long one then keep thinking, you're nowhere near done! When you plan for problems, it will usually mean that when those problems arise they can be taken care of quickly and efficiently. It's a great way to stay on schedule and in return, also means that you'll have more time to shoot.
You should have different backup plans to resolve each and every situation. It is a lot of work but it is your job as a director (and most likely producer) to maintain a kind of control that lets people know if they have a problem they can always come to you and expect an answer that resolves their issue immediately or near immediately. It will not only make you appear more professional but it will force an amount of respect from your actors and crew members.
The worst thing a director could say to anyone on his crew is, "I don't know." Even if you really don't know, play it cool and let them know that you'll have/get an answer for them soon. With all of these things in combination you will have a production that, from an outsider's perspective, appears to flows seamlessly even though you know better.