After you've closed the loop and notice you've had a jump (a complete pain in the ass, I know), just increase the magnification to say...400% and centre the area that needs to be fixed.(Fig. 1)


Fig. 1

To add to the selection (if the first mag-lasso cut off say, a finger), place the cursor inside the selected area (the cursor will change and become an arrow with a rectangular box)- (Fig. 2), press and hold the Shift key. (the cursor will change and become the mag-lasso tool you're familiar with, but you'll also see a + sign beside it).(Fig. 3)


Fig. 2

Start anywhere inside the selection area and click the mouse to activate the mag-lasso tool like normal (remembering to hold the Shift key down), then trace around the finger.


Fig. 3

To close the loop, continue back into the selected area until you connect the line to the start (you'll see the + sign turn into a circle) If done correctly, (Fig. 4) the cut-off finger will be re-attached. (forgive the gory image...lol)


Fig. 4