A friend and me were in a 2-man party looking to do the CoF dungeon. A 3-man party was already listed in the lfg tool, so I clicked the button to fuse our parties together. He got in, but I was dropped out.
What I guess happened is that between me clicking and the game actually processing it, somebody else must’ve joined that 3-man party, turning it into a 4-man party with not enough room for both of us. I think in such a case, the game should cancel our joining instead of putting one of us into the group but leaving the other one out.