lag and nothing more :p (i think)

 

This has actually happened to me quite a few times, I find that the only way to fix this ingame is to reload the chunks. So you have to run away from them far enough for the chunks that the other player is in to reload and then the player will be visible again.

F3 + a also manually reloads your chunks but I haven't tried this while in sg and then the glitch occurs

 

You were burning, can clearly see that iron sword at the bottom right hand corner of the screen...

 

Ah, yes.
I have experienced this too, just keep swinging lol

 

Out of curiosity, how do you know this isn't someone using an invisibility potion with the duration running out as you run away, or if it's actually a glitch? It could be someone with a bow that hits from far away, then comes up close invisible barefisted

 

Well in my case it was deathmatch and the user who was attacking me was not a donator and I also play with particles on so if it were someone using an invisibility potion I would have seen them. Another way you could tell that it is in fact a glitch is that you don't see a sword when they hit you and yet they do 3+ hearts of damage.
Using the screenshot from dj2000dj you can't really tell whether or not this is the same glitch but as he describes what happened it seems to be similar to what has happened to me multiple times.

 

This has happened to me and when I ran away just to explore the map and then I came back to the center the player was back

 

By leaving the center the chunks were reloaded allowing you to see the player again which is about the only thing that seems to work when this happens.

 

Well then I would suggest just attempting to run away from deathmatch and the other player as that seems to work. However it is more likely that you'd die in the process as I have many times.

 

I had this happen and it was just the two of us and I couldn't see him so he won...