When putting any two entities into a nether portal, or into different nether portals, from the same dimension consecutively, the game will crash. This isn't limited to a single nether portal, as if the first entity enters a portal, and the second enters another, the game will still crash.

To test:

1a. Create a nether portal either by obsidian and flint or /setblock portal.
1b. Alternatively, create two nether portals and use one entity for each, for the same effect.
2. Summon/push/drop/teleport any entity into the portal.
3. Summon/push/drop/teleport another entity (same or different) after this quickly (after a time has passed, putting another entity through won't crash the game).
4. Game crash. (See attached crash report)

Note: This occurs with players as well. The game will stay stuck on the downloading terrain screen and doesn't seem like it will change. To test:

1. Enter nether portal from overworld.
2. Enter nether portal quickly again from nether.
3. Enter nether portal quickly again from overworld.
4. Stuck on downloading terrain.