

These creatures can accumulate over time in the Nether not only is there no sunlight to burn undead, but without a player present, time will barely pass for them (15 seconds for each new entry) so they may not have time to despawn.

Portals built in the Overworld should be secured in order to prevent wandering monsters from entering the portal. Furthermore, a portal can spawn on a one block thick ledge or floor, or on a Soul Sand outcrop. There is also no check as to whether a lava source (created with the landscape) is destined to send lava flowing over the portal. Thus, a new portal from the Overworld has a disproportionate chance of being next to an abyss, lava lake, or netherrack wall. Portals try to avoid spawning over lava, in midair, or inside rock, but they do so by spawning nearby.

1.1 Portals can accumulate Overworld mobs.
