Day light savings transition days. When you skip ahead, that day has 23 hours in it. When you fall back, it has 25.

Also leap seconds: https://en.wikipedia.org/wiki/Leap_second

Also, any other change to the system clock. NTP will either slew or step the system clock to keep it synchronized with the upstream time source. And the user can explicitly set the clock, as well. See also assumptions 9 and 10.

What moskie explained, but got me thinking - if somehow your laptop always synchronizes with the current zone, and you are in a airplane flying east->west, or west->east?