What four common mistakes in estimating the WACC should be avoided?

Question

What four common mistakes in estimating the WACC should be avoided?

Answer 1

There are not too many places in the formula where a mistake can be made.  I suppose the most likely place is in the expected borrowing interest rate.  That would be 1.  Next would be the tax rate.  That can go up or down.  That would be 2.  Next would be the cost of equity.  That would be 3.  The market value of equity is always changing and changing rapidly too I might add.  That would be 4.