Years ago I had one of these mains-based clock... sometimes it become crazy and doubled the seconds count (it blinked at 1/2 second interval) for a minute or so...
In fact, mais based clock is in reality a pity, since in the reality, the mean frequency in practice is slight > 60Hz (or 50 for the 50Hz countries) in the long run, even with national operators dictating near 60Hz for entire interconnected system (for example, here in Brazil) and power plants need to sync with it perfectly or they are automatically disconnected (of course, due to electrical phasing mismatch/overload etc). Here also is common to a mains based clock to having 1 to 4 minutes/month added.
I have a DIY clock with 6dB passive plus 24dB/oct active filtered clock followed a carefully calibrated squarer to pick the mais freq, and even so, this time difference occurs.
I also have a crystal clock in the car that advanced 5 minutes in a month...
when I have some time I will sub the internal circuit for one from these home wall clocks that have far more accuracy.
Recently I built this vector CRT clock:
http://www.sgitheach.org.uk/scope2.html having a promising RTC; will be very happy if the RTC is precise as described in it's datasheet (about the time issue)...