diydrones/ardupilot — pull request #1148 by NullVoxPopuli (2014-06-16 14:24:40+00:00)
Title: Copter: Fix VTail mixing rear motor order (swap output 2 and 4)

Motors 2 and 4 were reversed for the VTail frame

On my copter, I've had to switch the ESCs connected to outputs 2 and 4 to get the quad to not yaw out of control.

here is another case where someone was experiencing the same thing (see comments):
https://www.youtube.com/watch?v=bDg1B12SxwA

Tim Anon experiences the yaw problem, and after upgrading to the latest beta release, he still has the yaw problem. After he switched outputs 2 and 4, his quad flies normally.