Boolean Union vs Difference Error - Swapped Behaviour I've been working happily with Blender all this week and last, performing hundreds of operations, many booleans, and getting good results in a rudimentary model for a small enclosure.   

Today I've renamed the file to v2, and started to make a couple of changes.  I'm just removing some material using a simple shape to create insets and a boolean difference operation.  Suddenly, though, the 'difference' is behaving like a 'union' and vice-versa.

I thought I'd forgotten something overnight, and tried it on two other simple objects in the same design, with the same result. I then opened a new design with just two cubes and tried it there, where it works without error.  Quit, back to my own design v2, and still the behavior is swapped.   

I've restarted Blender a couple of times.  Moving on to trying more renaming, rebooting tests etc.  Is this a known issue, am I missing something obvious? Is there a work-around that someone can suggest.   (I'm using ver 2.70 on Mac OS 10.8.5)
I had this exact problem about a week ago.  IRC told me that the face normals on one of my objects were probably backwards.  It turns out they were correct.  

You can turn on the display of face normals in EDIT mode using one of the icons in the Mesh Display section of the N pop-out panel.  

You can reverse the direction of the normals of the currently selected faces using the Mesh > Normals > Flip Normals operator.  There are also some "smarter" operations in that same menu.