Userforms. If two controls set each other's values, you can get into problems. For example:
[vba]Private Sub Control1_Click
Control2.Value = x
...

end sub

Private sub Control2_Click
Control1.Value = y
...

end sub[/vba]

The problem is that, in setting a control's value, you automatically call that control's _Click code. Windows VBA is pretty smart about this, but Mac VBA seems to get stuck in loops, usually resulting in Excel unexpectedly quitting. So be very careful how you set up this kind of code.

Call SubInModuleX. Unlike Win VBA, Mac VBA does not always seem to like calling a public Sub in another module unless you use
[vba]Call ModuleX.SubInModuleX[/vba]
Seems very random. Several possible error codes including 'Sub Not Defined' and 'Illegal Use of Property'.

Hey, but the good news is that I now have probably about 70% of my Win VBA code running in Office X now. So I'm getting closer.....