Occasionally you can't hit entities with anything

Was this in singleplayer or multiplayer?
If multiplayer, what kind of server?
If singleplayer, is your client modded?

I actually can confirm this, but since I found too hard to replicate, I didn't open a issue. But since then, I know WHEN it happens (while still hard as hell to replicate), so maybe you can find out why and fix it:

Situation 1 (most common to me): I have a large sheep farm about 30m from a nether gate. Sometimes (quite often actually) when I RETURN from the Nether, I go to get some wool as usual to stack up my reserves, except some sheep I can't neither kill or farm wool from as if they were not there. It also happened ONCE that two zombie pigman who spawned/teleported to the overworld where also impossible to hit. Quit/reloading solves everything.

Situation 2 (difficult to replicate): After sprinting from very far (300m+) towards my main town/farm, the same as above happened, with some sheeps being untouchable, others ok.

Hypothesis: Something to do with mobs loading improperly in a chunk when the chunk is loaded -thus when I return from the Nether (all the world must be loaded) or coming from far (also those chunks with the farm must be reloaded), this happens. This never happened to me when moving around from great distances were not involved, thus reinforcing my theory this is chunk-loading related. While I could not touch them, they responded normally to my presence by looking at me and even being pushed around, as if only mouse-related interaction where failing (hitbox related?)

Confirmed. Occurs on our server restarts and after just loading the mobs. They can be targeted again after them moving several blocks (usually like 10 blocks or so).

Happens with skellies in single player hardcore mode not modded at all. You can replicate by stacking hundred of skellies from a spawner in a 1x1 hole. The mob will visually seem to be passing thru a solid wall and he will wander around but he is still trapped. He can't hit you or vice versa - he is like a ghost. After a few seconds (probably a chunk update) the mob freeze and slide directly where he is supposed to be. It is only a visual glitch.

Semi-Visual actually, because the client thinks it is really there. If it happens on boats, you can stand on them. (But not get in them)

Is this still a concern in the current Minecraft version? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.

This still is the case occasionally, but it isn't that bad actually. They wont attack you, and you can't get them, so it is fine. Think floating squid and boat. They don't actually exist, they only do clientside. It isn't works as intended, but it is minor, so it could be "won't fix".

This pig had been on that block for forever so I decided to get it down but I couldn't touch it. I attached the video of it too. I do have optifine but it doesn't seem to be the problem since you guys are all having it too.

I was somewhat able to reproduce it: I've set my spawn with "/spawnpoint", spawned couple of skelis, let myself kill over and over and after each respawn tested if I could hit those. And indeed somewhat randomly was unable to hurt one or both of them.

Its so annoying for map makers 😞

This exact thing happened to me today. I was knocked into lava by a zombie, then I came back to recover some of my gear and found I couldn't hit it with my sword, and it actually killed me again (although this time it just ate my brains). I can't positively say it was the same zombie, though. Pokechu22's assertion that "they don't actually exist" is false... or he could be reporting a different issue.

Edit: I should add that the second time, when I was fighting the zombie I was cornered in a narrow 1x2 recess in a wall. There's basically no way the zombie could have just been in a different place than it appeared on my screen, or else it wouldn't have been able to hit me, so I don't think it is just a graphical issue.

I know the bug that I was thinking of was the one where they become different places, server-client.
That sounds like a different one. I know that if you have weakness (level 1), you cannot hurt stuff with your fist (you do 0 damage), so that may be related, although I doubt you splashed yourself with weakness to prepare to recover your stuff...

http://www.youtube.com/watch?v=i6fUAP8qR6Q
setspawn near mob you are fighting and die few times

Is this still a concern in the current Minecraft version 1.7.4 / Launcher version 1.3.8 or later? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.

Yes, happens to me all the time with my villagers in 1.7.5 still... I can't trade them sometimes. Happens to paintings and item frames too, and hostile mobs. I can't tell you how many creeper holes I had to plug because of this...

I'm also not using a mac computer, so the environment appears to be anywhere...

EVERY SINGLE TIME IN 1.7.9! Every time I die, I try to kill what killed me, and they don't take damage :/

Confirmed for 14w26c.

This bug appears to be caused by the client-side Chunks losing track of its entities, as far as ray tracing is concerned. Entities still manage to render.

Attached is a world that reproduces this bug. This demonstrates the bug for vanilla 1.7.10, but not snapshot 14w34d.

hey... this happens on wii u too! sometimes you cant hit mobs or players!