Hi Swegmang, that will happen if the objects you are booleaning are not closed solids. Normally the booleans figure out which pieces of each object to keep and which to discard based on what solid volume the piece is contained inside of. When your objects are not closed solids that won't be able to happen and it will work as more of a surface trimming operation.

You can tell if an object is a closed solid by the object type indicator, it will read as "Solid" if your object is a closed solid, if it says "Surface" or "Joined srf" it means your object is a surface and not a closed solid volume:




Where did you hear that it's "not fixable" ? Because that's certainly not true, you can build some surfaces and join them in to seal off the holes and make your surface object back into a closed solid again. But it is easier if you keep it as a solid throughout though. You can do that by using solid modeling functions (meaning booleans) to do cutting, don't use surface modeling functions like Edit > Trim and make sure you start out with a solid before you start trying to cut it.

You can also use the script here to highlight unjoined edges (also called "naked edges", that is edges that only belong to one face rather than being joined between 2 faces): http://moi3d.com/forum/index.php?webtag=MOI&msg=2798.6 . That will show you the specific edges that are open.

- Michael