I have noticd something strange lately. When I used to flip axis I hit S then X or Y key depend which axis to flip. but sometimes it keeps fliping it a wrong direction, or some strange inversion. What would cause this to work different from object to object? Even if I select a group of items and try to flip along Y axis all at the same time, some may flip right other’s get into strange rotations.
For the first question I can’t tell, but I have same problems sometimes.
For the second question I had that problem too and I solved this way.
1-Place an empty in the point with respect to which you want to flip.
2-Select things to flip and parent to the empty
3-Flip!
You’ll flip empty and all meshes correctly!
Hope this helps
Stefano
I use the rotation/scaling around the 3D cursor preferably ; that way I feel more in control of the result since I know around what precise point the flip will occur.
S+X always occur sideways relative to the window ans S + Y occurs up to down, this for the standard orthogonal views only (top, bottom, left, right, front, rear). Furthermore, the whole process acts regardless of the world coordinates or the object coordinates.
That’s strange, confusing and I have no explanation for such an unhelping way of doing things. I guess that Sharon and Arafat teamed up in programming this one.
I will try the parenting thing from now on. Just another hassle I don’t need though. I thought it was another bug, but I guess just poor coding on NaN’s part. This Scale/Flip operation I see in many tutorials I thought it was error free. It seems more hit & miss, but works best on meshes I think.
Thank you for the input, now I know it was not just me.
DAK: Always flip in “Edit Mode”, (F9). That way you will never have a problem. Also, do waht jeanamontreal says. Always flip around the cursor. With these two items, I have yet to have a problem.
Hope this helps.
BgDM
Yup! But this is if you have to flip a single mesh (or flip meshes one by one) if you want to flip many meshes parenting helps 
Stefano