(UNRELATED to my other ticket about witches, MC-48374.)
Nearly every hostile mob I've seen (since the 14w06a snapshot) did not notice me at all, even when I'm standing idle within their aggro distance, for about a minimum of half a second, to a maximum of about six.
To Reproduce
1.) Look for or spawn any mob that is initially hostile, besides all mobs I listed below.
2.) If you are in creative, switch to survival.
3.) The mob will take slightly longer to notice you then it would, with slight time variations each time.
The following mobs do not experience this issue:
-All neutral mobs (Enderman, zombie pigman, wolf, spider and cave spider (daytime), wither boss itself)
-Wither boss side heads
-Ender dragon
-Slimes
-Magma cubes
-Ghasts (They can't see you until you get almost eye level with them, but when you do they immediately shoot, I made another ticket about this)
The following do experience this issue:
-All other initially hostile mobs (Zombie, creeper, blaze, guardian, silverfish, shulker, etc.)
NOTE I would like to note that it appears to affect mob-to-mob interactions as well. Tested with iron golems, snow golems, and mobs that now attack iron golems. That is, all of them have a short time before noticing each other. Happens with zombies vs. villagers and guardians vs. squids. They react as they would with the player.
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I've noticed it with all mobs.
Yes
I can confirm it too and this bug still appears.
Paul Smith, you had an issue about the ghast.

Doesn't seem to happen with Zombies and Villagers (but then again Zombies can detect Villagers through blocks).
The mobs have a detection range. When you stand outside that range they won't detect you and it won't turn his face to you.
I was within their detection range for a max of 6 seconds until they noticed me. If you look at the screenshots, you will see I'm directly in front of them. Test it yourself, I've seen it in multiple YouTube videos and on different computers. And I have tested zombies vs villagers and guardians vs squid, and both have the same results.
But I have tested it in a server and I don't notice it. Only in singleplayer.. I have tested it with ghasts.
Thank you Zi-Wen Lin.
I just want to put this on here. This is the ideal video for demonstrating this bug in action:
https://www.youtube.com/watch?v=S0_Grxp3l-U&list=UUnkNLtrV6ZNWMK5ytpFGg8g
It is from a semi-well known YouTuber named Tozatop. I want you to note some key points in the video, and notice how the surrounding mobs do not attack him instantly and he even gets some kills without the mobs being able to react.
2:02 creepers
3:04 skeleton
7:38 spider
8:07 skeleton and creeper; he acknowlages the long aggro delay at this point
8:40 zombie
Please read, watch the video and confirm this bug. Unless "community consensus" means the same thing, I wouldn't know.

Also affects 1.8.1-pre2 as well.
Still present in 1.8.1-pre 3.

Still in 1.8.1.

Confirmed for 1.8.3

Confirmed for 1.8.7
Still in 15w35b.
This may be the most significant bug that is currently in the game. This should be a top priority bug.
Confirmed for 15w50a.
I'm sorry, but can somebody tell me why this is an intended feature? Because it came across as more of like a bug to me.
Humans have delay in response time so that could be the reason why mobs also need delay in response time.
But if it works as intended, why did they consider it as a bug from 1.8 snapshot until now? Almost two years have passed since then...
I liked singleplayer more when mobs see you immediately.
Zi-Wen Lin, it could be a logical reason, but I would like to have a direct answer from Mojang. An explication of why they consider it works as intended after two years treating it as a bug.
(sorry if I've written somethign wrong, my english is quite bad)

@@unknown It being unresolved for 2 years does not equate to Mojang themselves considering it a bug.
Can some Mojang employee please tell the public why this is considered working as intended? Everyone would like to know.
I'm not Mojang but I'm guessing it's because mobs do not have god-like reflexes and/or eyes on their back.
Maybe their brains are just not smart enough. After all, they actually attack you for absolutely no reason, and creepers commit suicide on purpose.
and also wouldn't it be fun to run up to a mob and backstab it, I do it frequently while mob fighting. this is a feature.
The greatest reason for this bug would be that first of all, ever since 1.8, although it was said that the game has been made a lot lag-free, there is still lag in the client. This mob lag actually doesn't happen on a fast computer or on servers. It does happen on servers but if the server is fast, then there is no problem. The biggest reason for this lag I guess would be because of the higher number of entities. If there are armor stands, command blocks, redstones, mobs, etc. then this can be seen clearly. Sometimes in survival, the mobs stop moving but we can move and do anything. BUT NOTICE: When we attack them, they won't get attacked. And also, if we wait for the game to work, all of a sudden, the game goes too fast and then the mobs attack us 5-7 times in 1 sec. Also I guess if we check the logs, sometimes it says the message like: "Can't keep up! Did the system time change, or is the server overloaded? Running "x"ms behind, skipping "y" tick(s)".
The running of "x" minisecs behind forces to skip some ticks, which causes the mobs to attack all of a sudden. This only happens in SinglePlayer, or MultiPlayer if the server is overloaded or can't keep up. But if this happens in MultiPlayer, it won't show in our logs. Rather it shows in the admins log. (Well, because HIS server is lagging lelz)
Confirmed for spiders