While flying your latest design, does your plane suddenly yaw as if some giant invisible hand forcibly slapped the side of the empennage? Depending on your design, this unwarranted push may or may not send your perfectly flying plane into a sudden death spin. luckily, I have found the solution.

The solution is to build the stabilizers out of structural wings (Wing-2) and use XML to enable control surfaces.

Alternatively, you can attach the stabilizers onto structural wings if you do not have mod support

Attaching stabilizers onto a flat surfaced fuselage piece also seems to work

To replicate this bug, you must first attach a vertical stabilizer (Wing-3) directly onto a curved fuselage piece. Then, during test flight, perform a series of wild maneuvers (rapid rolls, change of pitch, change directions). At some point the aircraft should suddenly yaw to a side.

My guess is that this bug has something to do with the rigidity/flexibility of regular wing pieces (Wing-3). During wild maneuvers, calculation errors cause the wing's relative position to shift. Then, subsequent flight calculation takes this shift into account. This momentary imbalance is what creates the sudden yawing force.

Structural wings (Wing-2) don't have the rigidity problem that regular wings (Wing-3) have. Therefore, this bug should never manifest itself if structural wings are used for everything. In conclusion, something funky is going on with them regular wings.