unwanted plane showing up in 3d view

[Keyser Soze]

Thanks for your help...i had two ceiling heights..10' for the covered entry court outside and the entry inside and the room to the left was at 9'..this should not have created a problem but it did..i fixed it by changing the inside entry to 9' and moving the entry wall and then changing it back and moving the wall back...very weird...this is not the first time i have seen this triangle plane appear between rooms?