sooo... I know this is soooo many years later.. but I had this problem today. the house I was in was fine, until I misplaced a gnome box, or perhaps it was because I put buydebug in testingcheatsenabled... anyway, this house was generated in town and I bought it (I'm not ready to build just yet) on a corner lot. it had a diagonal entryway. I was rearranging furniture in a bedroom atop this diagonal room, and noticed every edge had these squares (only they were halved as in triangles because the room was diagonally placed on lot). after I had tried resetting sims, lots and town through nraas, I find out a piece of furniture is glitched. I noticed the upstairs room.... for some reason I thought to pull the whole wall in and then put it back out again. that worked! for the upstairs. it appeared along different places on the 1st floor on one side of house. i tried to pull walls in and out again, (I had game on pause while I was trying to fix this issue so I didnt realize door was open) but a sim was coming thru a door. unpause. sim is far away from door.. but it remains open. I ended up having to delete the door, pull wall in then out again, and it went away 100%! so that's easy but it is only 5 or 6 years late and of course, only 1 solution for 1 of the 110 potential glitches that could be causing this one problem...