I've had this happen twice.

Somebody hosts a DM game and I join. They scroll through the maps for a while, finally pick one and launch. When I launch, I end up finding out that I am not on the same map as them.

It caused quite a weird sight for both players. Sometimes my opponent is underground because of elevation differences other times they are very high above the ground, just hovering like a Fury.
This happened VS Firestorm29. I was on Armageddon and he was on Dragon's Mouth. It was actually quite funny, I was laughing so much. I could shoot the ground and deal damage to my opponent. It was freaky.

Firestorm29 and I played for a while, the whole game was a complete joke being on different maps.

It happened again VS COMMANDER GREG. I was on Mars Rally and he was on Warzone.

In this game I rejoined and got to Warzone map instead of Mars Rally.

The 2nd time wasn't so funny. At that point it was annoying.

Has anybody else ever seen this? I can't be the first.