I had this happen to me the other day after 3 respawns from invisable zombie deaths, I was very fortunate to meet up with a player who was able to contact the server admin. Server admin forced server restart and did not find any invisible zombies after that. (more info) Version 0.48 stable. Was on a low population server 9 players. The invisible zombies seemed to apear about 30 minutes after planed server restart, this server has a 4 hour restart. (Have not encountered any other invisable zombies since.)

This is a rather an unnerving bug, the zombie sounds seem to be getting worst, used to be just chirps,grunts,small sound clips. Now hearing complete zombie sound clips, awareness of player, charging, attacking. Combined with the other sound bugs, my level of paranoia is thru the roof. (note:) stable 0.48, seems to happen on low pop. server after 1hr of play, on high pop. server within 15 min. of play. (time of last server restart does not seem to be a factor.)