Hi FDP yeah the problem seemed to be that one particular bad surface shown above.

In the attached file I have deleted that one surface, and built a new one by extruding one of the remaining surface edges over and then joining the pieces together to make a solid again.

With that face fixed the boolean difference now seems to be ok.

If you happen to have a previous version of this object right before that rounded spot was constructed, if you can possibly send that to me at moi@moi3d.com that's the step that did something wrong that would be good to fix up.

- Michael