Basically, any redstone torch or repeater that switches state often will lock itself into one state, (usually off, but not always). People have observed that this occurs more often when the /time set is being used, but it also occurs completely independantly of that command. As a result, clocks/pulsars/etc will jam up in a couple of minutes, with my personal experience being the faster the clock, the faster the seizing. Destroying and replacing the torch doesn't fix it, btw, only known cure is to wait a full minecraft day without time setting, and even that isn't 100% successful.
This also has the unfortunate effect of making circuits much larger than one or two torches very prone to jamming. For example, the largest circuit currently on the server is the one under the first station (just east of the central keep). That jammed three times as I built it, and once some time later (the greater amount of jamming initially was because there is a lot of state changing happening when connecting circuits and testing settings), and it's not much more than a couple of torches used to propogate and invert the signal. Because traffic is fairly quiet however, it doesn't seize too often, and it's relatively easy to repair. The GC station on the old map, by contrast, used to jam every couple of RL days, and was a monster to debug.
Buttons, levers, and redstone wire itself don't seem to jam, so basic systems using those are generally fine, but anything greater generally doesn't last long. More than anything else, I wish Notch would fix that bug