Recently I needed to remove a door from a small house mesh so that I could replace it with a standalone door object. I had a surprising amount of trouble removing the door from the original FBX mesh with the CAD tool I use, so I hacked together a Unity script that could let me do that and more.
I wrote a Unity Editor script that lets you explode a GameObject into one child GameObject per triangle, and then recombine those children back into the parent GameObject. While exploded, you can transform or delete each individual triangle on a need-to basis. When they're all combined back into the parent GameObject, your changes will stick.
You may think "Well that's absurd; seriously just get your artist to fix the door for you or buy something on the Asset Store to do it!" Well my artist contract has long expired, and I knew I could whip something together in a few hours anyway. While I was at it, I figured I'd share the script with everyone developing games with low polygon models. I hope you won't need it; but if you do, then I hope it helps...just don't try to use it on a high polygon model!