$work had thousands of full custom, dsp-heavy, location measurement hardware devices widely deployed in the field for UTDOA locating cell phones. It used GPS for time reference -- if you know your location, you can get GPS time accurate around the 10's of nanoseconds. GPS also broadcasts a periodic almanac which includes leap second offsets: if you wanted to apply the offset to GPS you could derive UTC. Anyway there were three models of these units, each with an off-the-shelf GPS chip from one of three location vendors you've probably heard of. The chip firmware was responsible for handling leaps.
One day, a leap second arrived from the heavens. We learned the three vendors all exhibited different behaviors! Some chips handled the leap fine. Some ignored it. Some just crashed, chip offline, no bueno, adios. And some went into a state that gave wildly wrong answers. After a flurry of log pulling, debugging, console cabling, and truck rolls, we had a procedure to identify units in bad states and reset them without too many getting bricked.
It seems the less likely an event is to occur, the less likely your vendor put work into handling it.
