I get the opposite happening. I can run past some mobs to the point they are just out of my vision, then if they move in my direction and I go back towards them there will be 2 of the same one. One will disappear after a second or two.
I've experienced this glitch and the on Old Man posted.
The One with the mobs also happens with players for me. It's like they have some residual trail that goes away after a second or two.
With Old Mans, some items for w/e reason seem to vanish at times. When you cursor over them they appear but move your cursor away and they vanish again. Relogging has fixed this for me when it happens (Generally something that happens on log in haven't noticed it at other times).
Both could be related I think, but probably aren't lol. To me it seems more of an issue with the server relaying information to the client as to what is there and tracking it. It's like it hiccups at times, or simply forgets at others oO.
I am far from an expert so this is nothing more than me rambling.