Implementation specific. It looks like it is a bug in the Linux kernel with how it adjusts the time. It is possible that Windows, OS X, and other BSDs will be affected by a similar bug, but that would be coincidental as the bug is not due to ntpd but rather how the kernel handles a request that ntpd generates.

More specifically, there is a condition in which the kernel tries to insert a leap second and, in doing so, attempts to acquire the same lock twice causing the spinlock lockup and (effectively) halting the kernel.