Looking at the map of Frontier, it seems that someone has journeyed far in the Nether, made a portal, but on travelling through has found that they are beyond the border and would have been instantly teleported back within. However, the Overworld chunk where the new portal was created was still generated, and it is a mushroom biome.
While I am pleased that this proved we can generate all biomes, I must warn players that such exploratory portals are not encouraged. As this player found out, it does not help when you are beyond the border. Also there is no guarantee that the portal will not be generated in the middle of someone-else's construction work. That is why we have a guideline in our Code of Conduct
- "Construct nether portals only in the "Overworld", not the Nether".
Furthermore, this needlessly expands the size of the map, which is one of the reasons why we have the border mod in the first place. If this happens regularly we will have to start trimming the world to remove such outlying chunks. That is an administrative annoyance I don't want us to have to do. Likewise, I don't want to have to add a border to the Nether.