I have witnessed this on one occasion.

It happened to me yesterday after being hit by someone (with a shovel)

This occured twice for me, both times after conflict. I do think it has something to do with becoming unconscious after combat. I fell out of a building and it didn't occur; I got hit in the head with a baseball bat and it did.

agreed, some dead players will slide aswell

I get this too

Seen this happen every single time. When unconscious characters slide over the floor until they respawn/die. Sometimes they also stand in an upright position until they respawn/die.

Agreed, this happens after a melee knockdown.

This has happened every time I am knocked unconscious by a zombie and briefly regain consciousness even though I cannot move because I am sliding.

The only way to not die due to inaction is to log out the moment I come to, and when I log back in I can once again stand/move/bandage/fight back.

Very annoying when it happens because it is usually a death sentence.

This may be a dupe of #87, the cause is likely the same, but a slightly different symptom is reported

Definite dupes:
#279
#319
#542
#679
#698
#2081
#2281 (has video)
#2441
#3116
#3144
#4092
#4185

Possibly related:
#372

Still happening in 114557...

Still..