Recently, the mob spawning algorithm was changed to no longer require air blocks for mobs to spawn in. As a result, mobs can now spawn in nether portals. Upon spawning, these mobs are transported to the nether (creating a short lag spike), where they will stay until a player arrives. If a player doesn't use their portal(s) for a while, they will be greeted with enough mobs to kill them once they do.

This mainly affects slimes as other mobs require a light level below that produced by nether portals.

To reproduce:

1. Create a flat world
2. Find a slime chunk
3. Place 16 16-wide max height portals on it (so the whole chunk is covered in portals)
4. Place half slabs on all other chunks and on top of the portals to prevent other mob spawns
5. Light the portals
6. Wait a few minutes
7. Go through one of the portals and observe the slimes that have appeared by the nether portal on the other side