Thanks guys.

As far as I can see the hidden objects are not a problem. When you hide everything and just leave the "cup" object visible, you can see there are several duplicated "patches". ( when you select and delete some you'll see there's another or sometimes more hidden under it )

I recreated most of the problematic areas, and it looks like the problem is fixed. I'm quite new to Moi and I suspected it to be more forgiving with booleans and trims. But maybe it was just sloppy modeling ;-)

I'm still curious how this can happen though.

Thanks again,

-Ronald