When entering a Nether portal, one or more stray chunks will be generated in one dimension at the coordinates for the other dimension. This is most commonly seen when stray chunks are generated in the Nether at the Overworld coordinates.

To reproduce:
1. Create a new world of any kind, eg: default or superflat. Enabling creative mode is recommended to make it easier to test but it can be reproduced in survival.
2. Teleport some distance away from spawn, say 5000 blocks away
3. Create a Nether portal and enter it.
4. Return to the Overworld.

Sometimes, when moving through the portal, one or more stray chunks will be generated in the Nether at the Overworld coordinates.

This also happens in reverse: stray chunks are also generated in the Overworld at Nether coordinates.