Terminology:
Packet - some bytes in specific order those are transferred through TCP connection between client and server. Read more about it here https://wiki.vg/Protocol .

Packet id - an integer that prepend of packet content. 

Issue:
it changes from version to version and somethimes even in minor versions it changes. It put a lot of problems to support some packet stuff on different versions.

Edit: 
This issues can not be reproduced, because it is an internal problem and affects only java plugin/mod development. It doesn't mean that it is not necessary, though. Only way to fix it, it is keeping id equality in compare with future versions. I provided an information from https://wiki.vg/ which demonstrate that a developer insert packet right into start of packet list and it is shift following id of packets.