Numerical Misdirection.

You can easily become a numerical klutz by assuming you have better numbers to calculate than the ones that are most commonly used across an entire field. 

DCF to Death.

You have to make assumptions if you’re unsure about the future prospects of a company. Just because you assert some perpetual growth rate, doesn’t mean it’s going to play out as expected. Even a margin of safety is a “I’m too dumb to know” boundary.