Payroll "forgot to add" my overtime

So I worked 45 hours last week and I realized that my paycheck was a little low, from looking at the paystub they only paid me for 40. I checked the timesheet they used and it corrected showed the 45 hours. I've been an employee of this company for a long time and I like most of the people involved, but my question is: how would you take it? Is it possible it was a mistake or do you think they were trying to rip me off? What's your first inclination?