i'm working on my mmv, and if i mask the mouth it seems to be fine in the preview window, but after rendering the masking line will show up.
it helped a little bit when i made the section little bit bigger and blurred the edges, but in some parts i cannot do that and the harsh line will show up.
what causes this

for example if i mask a hand. and then fingers separately. then i move the fingers the way i want. but after i have rendered i can see the masking
pls help