Kind of a weird one, if you like a nether portal on the side of the obsidian frame, the portal will often times suffocate you, and this can even cause the portal to misbehave and send you to the wrong location/link up to other portals.

Lighting the nether portal on the bottom of the obsidian frame, solves all these issues. This even continues to happen after breaking the portal, and lighting it on the side again.

This is a rare one to reproduce, but it does happen on multiplayer worlds a fair amount, since lighting the portal on the side is common practice.