I am experiencing this problem on a house called Celebrity Mansion that I downloaded from the Gallery. Everything looks normal, my sims go up and down the steps just fine so no routing issues. It is just a cosmetic problem as far as I know but it makes decorating for the different holidays a real drag.
The decorations are from using the attic stack to select decorations for the outside of the house.
Edit By Crinrict: Added pack tag to title.
