Another UX issue I've seen with the iPhone calculator:

My wife is a nurse and uses the iPhone calculator to calculate doses. She performed a calculation like

  2.25
  /
  1.5
  =

It did give the correct answer of 1.5. However, there was no animation or blanking out of the display to easily distinguish between the operand of 1.5 and the result of 1.5. She thought it was a bug and wasn't giving her an answer. I always thought it was a little scary that such a horribly designed calculator was being used by so many medical professionals.