20th Fret Treatment
A common mistake is cutting the 20th fret slot all the way across the fingerboard. If you do this you will end up with a tiny slot on the bass side. You can add a tiny fret there and it looks ok. I used to do it. But its not the norm. The problem is that there is so little wood after the fret that most of the time it will break off. It can be a pain to repair since the fret will just keep pushing it out. I got tired of doing this and realized I was probably the only one putting a tiny fret there.
To avoid this you should only cut the 20th fret slot half way across the board. I usually stop just before the entire blade disappears under the fingerboard. But fear not, if you have been over zealous and cut all the way through there is a good fix that is undetectable if you do it right.