The bug/glitch is found in the following versions: 1.7.2, 13w48b and 13w49a (both Client and official/standard minecraft server)

This is what happens:

Whenever a player gets killed by another player (tested using a diamond sword and empty hand), and the subject chooses to relog, instead of respawn, the player becomes invisible to others and immune for PvP damage.

To see the player again (and to be able to deal PvP damage to him), the simplest solution is to kick him from the server.

If the player isn't kicked (because nobody noticed him becoming invisible) and the subject doesn't relog, all online players should relog in order to see the subject again and be able to deal PvP damage.

If i find the time i'll try to make a video of this: http://youtu.be/1tkWyaSp_Js

I find this very serious and alarming, as i happen to encounter this on a daily base, dozens of times and has a severe impact to the gameplay of others.