We derive an expansion for the (expected) difference between the continuously monitored supremum and evenly monitored discrete maximum over a finite time horizon of a jump diffusion process with independent and identically distributed normal jump sizes. The monitoring error is of the form a 0 /N 1/2 +a 1 /N 3/2 +· · ·+b 1 /N +b 2 /N 2 +b 4 /N 4 +· · · , where N is the number of monitoring intervals. We obtain explicit expressions for the coefficients {a 0 , a 1 , . . . , b 1 , b 2 , . . .}. In particular, a 0 is proportional to the value of the Riemann zeta function at 1 2 , a well-known fact that has been observed for Brownian motion in applied probability and mathematical finance.