Flipping both Vertically and Horizontally.

[zw]

While testing out the new features I noticed that V + H didn't flip the painting about both axes.

I probably don't have any use for flipping about any axis personally, but since you added the other two by themselves, why not combined as well?

[AndyRage]

The backslash key '/' rotates the canvas by 180 degrees (which is equivalent to a V + H mirror)

Really, the mirror flip keys are just to help spot errors in proportion by giving you a momentary 'change in visual perspective' of the work you're working on.
It's like a painter looking at his painting in a mirror to help him break his visual perception - seeing the painting anew.

[MattRage]

Though if I'm honest (I know, not generally a very commercially viable proposition) the reason it doesn't flip both is because I didn't process both keys simultaneously :)