Is this really a Tiger OS question (the forum in which you posted)? Do you want us to request it be moved to a more appropriate forum?
http://forums.macosxhints.com/archive/index.php/t-31161.html
Error code -50 is a generic code meaning there's some sort of communication difficulty.
You might want to provide more detail. I don't know from what exactly you are transferring and to which device when you encounter this message. You say these are backups from the Macbook Pro (you should always fully specify "Macbook Pro" because there is just a "Macbook" model and there is just a "Mac Pro" model), but are these on external hard drives? If they are, it could be the external drive is having problems too.