The older version of calculators have several “cancel” button (C, CE). The CE button will only clear the last entry but does not interrupt with the current computation. For instance, if you enter 2+1 and then press CE, it would show 0, but the calculator expects a second operand for the addition. (2+ is still there in memory). So to ensure that nothing remains left in memory and to start a new calculation we often see people hitting clear button couple of times. It is not due to a flaw in the calculator’s design.