straight then you can simply go over this area again to even it out. if you do go onto the finished ceiling with the wall paint, wait till it has dried and then do the same process with the ceiling paint, to even up the line again. you don't need to do the whole ceiling again, you are just "touching in" the bit of wall paint that has spread onto the ceiling.
don't be tempted to use masking tape as all that will happen is the paint will creep under the edge of the tape and when you pull it off, some of the previous paint will come with it and 