Leap second - possible problems?

Hi guys,
I'm dealing with really non updated machines (RHEL 5 - 6 and older) with uptime more than 3 years and I don't have a clue, what can happens if leap second occurs...
All machines are using NTP (but original source of time sync is unknown) and all have tzdata packages from 2012. Kernels are newer than affected and described in article. According to RHEL Article there are several workarounds.

1. Using NTP without updated tzadata - NTP leap second flag is not distributed >> In this case, is leap second is ignored?
2. Not using NTP without updated tzdata >> Also, in this case leap second ignored?
3. Not using NTP with updated tzdata >> Leap Second will be successfully carried?
   3.5 Why is neccessary to use /right/ packages?

What can be consequences in cases 1 and 2?

Thanks for replies! :)