This is the second time I am posting this as I was unable to post the screenshots above the first time. I apologise.
In the above link, the tutorial has a step a minute before the end in which he completes an extrusion with a clipping mirror modifier. At this point, the vertices at the ends of the semi-circle on the bottom of the anvil become locked to their actual vertex locations, instead of being near them due to the shade smooth and subsurf mod. I am unsure why this is occurring. Any help would be greatly appreciated.