By far the most common computed column issue that arises is the "divide by zero" problem.  To avoid the problem, add a check for zero when entering a formula that will divide two columns.

Example

IF([Qty]!=0, [Amount]/[Qty], 0)

Explanation

This formula will look at the amount in the [Qty] column. If is it not 0, it will divide the amount by the quantity. If it is zero, it will enter zero in the computed column.