Check if shape is in specific location in slide

[John Wilson]

This is kinda hard to explain but because you are changing the order of slides within the loop you are moving some "target" slides twice. eg the original slide 2 becomes slide 7 when slide 4 moves and is moved again. You will need to start at the end of the presentation and work backwards
using something like

Sub sort()
Dim i As Integer
Dim z As Integer
'if slide has a shape at 156 and 516.75 then move the slide to the back of the presentation. These slides are referred
'to in comments as "facing pages"
For i = ActivePresentation.Slides.Count To 1 Step -1
    For Each sh In ActivePresentation.Slides(i).Shapes
        If sh.Left = 156 And sh.Top = 516.75 Then
            'move slide to end (or end -z)
            ActivePresentation.Slides(i).MoveTo ActivePresentation.Slides.Count - z
            z = z + 1
        End If
    Next
Next

This would of course give you 3, 2 ,1 but should be a starting point (note code has been edited so the order is now 1,2,3)