What I expected to happen was...:
When my friend was joking with me by placing Obsidian at the exits of the Nether Portal (in the Overworld), and i relogged (logged out and in), the portal was "Defect", it didnt teleport me, it didnt give a ''Whooshwshwshwshw'' sound (it did give the normal Nether portal sound), no Nether Portal effect (Spinning Purple Circle/Nausea), nothing.

What actually happened was...:
Nether Portal didnt work as it should.

Steps to Reproduce (Survival):
1. Create a Nether Portal.
2. Go to the Nether.
3. Step into the Portal until you are back at the overworld. dont move if you arrived!
4. Place a block (I suggest something weak, like Sand) at the exit(s) of the Nether Portal.
5. Leave the World.
6. Log in.. aaand... your stuck!