Preamble: Ever since the most recent maintenance patch, my PvP guild (as well as several others) have been having an epidemic of random hard-crashes of the client. We have observed that they typically occur out of combat in Cyrodiil and that they typically take down 2-4 players that area nearest to the person that originally crashes.

Fast Forward: We are now confident that double-mounts play a role in the crashing, either as a direct trigger or else as a mechanism that greatly enhances the chance of a crash occurring. It is also reproducible, as we tested last night, with our only crashes of the evening coming when someone whipped out the new Dwarven Spider double-mount and then somebody mounted using it as the second rider. It appears to be the moment when a second rider is added that the crashes occur.

I recall that the long delay of the double-mounts was due to it conflicting with the anti-cheat protections run on the game. Perhaps there is something similar going on here? In any case, I sincerely hope that this receives some developer attention because, with all of the new Arcanists with slow mount speed running around, not being able to make use of double mounts is somewhat of a problem.