I have a 7x7 portal in the nether with a small closed area without light. The other portal (overworld) is in a mushroom biome and also closed off with dirt so no mobs could get it.

Sometimes 2 creepers and 1 skeleton randomly spawn in the other side of the portal (in the nether) when they couldn't possibly go though it. This has happened to me twice already.

It's hard to recreate and doesn't happen often.