are u talking about mobs? well its probably due to the large amount of xp grinders being used, therefore people overloading the mob cap, making the spawning of mobs slow
So when the total amount of mobs on the server reaches roughly 150+ mobs, naturally-spawned mobs don't spawn.
Exactly that is why players with Natural spawning ringers should install an off switch of such may it be a lava blade of internal lighting to prevent mob spawns...
I think the limit's been reached, but not because of grinders. Last week I was working on a blaze grinder because of the funny way they've been behaving, and tried clearing them out by disconnecting. I was gone maybe ten minutes, then when I came back the ones that had not been moving were still there in the same places. They should have despawned. I've been watching my debug screen at a skeleton grinder since last Tuesday and I've always got about 30 entities I can't see. We've even had a server reboot with maybe me and 6 other people coming back on. When I came back, all my skeletons were still in the grinder and the entities were still there. If people had had grinders that were full the reboot should have cleared them and I was back before they could have possibly filled back up again, but still no mobs spawning. What I think is happening is that across all the servers mobs aren't despawning normally and we've reached the cap that way. Mob behavior like what we're seeing in the blaze grinders is a secondary problem, I think. Since it's persistent after a reboot it can't be grinders and as far as I know MC doesn't store information about what's spawned in a chunk so it may be software. I'm guessing that Justin either applied a normal patch before he left or was experimenting to try and fix the grinder/mob cap problem. Once he's done moving I expect he'll be looking at the login problem first. Hopefully he knows exactly what's causing the mob problem and can fix it.
When you leave a chunk or the vicinity of a mob they despawn, at least they are supposed to. This problem is happening on all the servers all the time now even after all the reboots we've had. To do this we'd need some really dedicated people with super fast grinders.
Also worth noting that updates are only applied to chunks within a 16-chunk radius of a player. Someone could have logged off with 500 mobs and it shouldn't impact anyone else since their chunks aren't being processed. The other interesting artifact of this is that monster-spanwer monsters will actively pursue players while naturally spawned monsters seem to be mostly passive unless you attack them first.
ha! you guys are so dependant on mobs to get xp and goods! i use the good ol' giant chicken trap for my xp. i just get tons of eggs and then throw them in a maturing cell so they grow up (20min usually) then i slaughter them. it lags a lot if you are close to it, but the payout is great. i usually get an inventory full of eggs (they only stack to 16) in 15 minutes. chickens are not acting up at all, so its a great source of xp.
The zombie spawner I found has a gravity trap on it, however, it does take some bouts of afk-ness to fill the receiving platform.. I try to clear it when it reaches 100 which usually takes 15-20 minutes. The weapons and tools and iron are just an added bonus.. :3
