Wierd flaws not likely to be encountered
I discovered one of these on the operating system I worked on in the 70's. Turns out that the calculation of 'leap year' is done when the time was entered on the console (before CMOS clocks). The problem would happen if one entered the time in December of a non leap year (like 1975) and if the machine ran continuously till February 29th. Since the determination of leap year was done in 1975, and 1976 was a leap year, it wouldn't register.
Of course the likelihood of the machine being up for that long was sooo remote, they didn't consider it a bug. The remedy is to just enter the date some time in January, and go from there. There was really no need to reboot. Sadly the machine was sold for scrap in 1983 after a bankruptcy.