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)