I found the old filter didn't have the o-ring on it when I was cleaning up the mess. I hand tightened the filter quite hard and don't see oil dripping. Is it a potential disaster if I just carry on and ignore it? What kinds of problem might occur in the future? If I decided to fix it, can I take it off without draining all the new oil? It's a ford modular 4.6L and the filter is placed horizontally.
Yes, this is a potential disaster! With sufficient oil pressure, it can blow the extra ring right out resulting in a massive fast oil leak. Trust me, this only has to happen once for you to make sure it never happens again.
If you are lucky, as I was, it will happen very quickly (while backing out of garage after oil change) and you only have to deal with embarrassment and several quarts of oil on the floor. I hate to think what would have happened if my wife were driving down the highway when it happened - new engine I suppose.