wrong iso choosen

[scars]

beginner question... 
I have a kodak iso 400 black and white film loaded in my mx. stupid me
forgot to set the iso to 400 and let it on 100. I realised my mistake
after about 12 pics taken. what effect does this have? is it better to
take the rest of the pictures with the correct iso setting or is it
better to use the same iso setting for the whole film?
sometimes automatic cameras do have their advantages -_^
bye Katrin