Hi David, probably it's getting confused about these various regions where you have overlapping surface area:



Those areas where surfaces are overlapping each other with some shared common area tend to be more difficult for boolean operations to handle, but usually with planes it tends to work ok, I guess not in this particular case though.

Which operation are you trying to do - do you want to find the intersection of these (just the inner part only), or the union of them with the long parts still sticking out?

- Michael