Mantis#2423. Thank you kindly, Godfrey for a patch that:

Included patch fixes error: Z and W terms in the quaternion were 
swapped (ZERO_ROTATION is <0,0,0,1>, it was checking for <0,0,1,0>).
There is an issue with older prims: it seems their default sit 
target was not always set to ZERO_ROTATION;

1 files changed, 2 insertions, 2 deletions

diff --git a/OpenSim/Region/Environment/Scenes/ScenePresence.cs b/OpenSim/Region/Environment/Scenes/ScenePresence.cs
index 73bcfd8..fb9c29c 100644
--- a/OpenSim/Region/Environment/Scenes/ScenePresence.cs
+++ b/OpenSim/Region/Environment/Scenes/ScenePresence.cs
@@ -1230,8 +1230,8 @@ namespace OpenSim.Region.Environment.Scenes
 
                 bool SitTargetUnOccupied = (!(avOnTargetAlready != UUID.Zero));
                 bool SitTargetisSet =
-                    (!(avSitOffSet.X == 0f && avSitOffSet.Y == 0f && avSitOffSet.Z == 0f && avSitOrientation.W == 0f &&
-                       avSitOrientation.X == 0f && avSitOrientation.Y == 0f && avSitOrientation.Z == 1f));
+                    (!(avSitOffSet.X == 0f && avSitOffSet.Y == 0f && avSitOffSet.Z == 0f && avSitOrientation.W == 1f &&
+                       avSitOrientation.X == 0f && avSitOrientation.Y == 0f && avSitOrientation.Z == 0f));
 
                 if (SitTargetisSet && SitTargetUnOccupied)
                 {