CompuServe Messages

I hope someone can help so here goes … When in the 3DEditor module and attempting to Subtract one object from another. In my case I'm trying to cut windows into a wall. Every now and then the resulting 'hole' has a stray face across part of it. Erasing this stray face seems impossible no matter how many times I select it then erase it. Is there any way I can cut accurate sqaure holes in a wall without this 'stray face' effect ? Thanks for your attention, Chris Inman.