See the picture below.
The light blue represents the entire tallis gadol. The dark blue is where the corners are. Next to the bottom left corner there is a piece of the tallis gadol (represented by the white part) that is completely missing.
Can one still wear this tallis and make a bracha? If it can't be worn if one would put a piece of cloth to fix it would it be a problem of "t'aseh v'lo min ha'asui"?