Because so many of you offered such good suggestions on the source of the invisible wall that appeared in my Linden Home, I wanted to pass along the explanation offered by SL Support. The wall covered a corner in my home, and in that corner sat a potted plant. I had resized/enlarged the plant when I put it there. Support said: "If you resized the object, it may have wound up resizing it's bounding box. Bounding boxes are, essentially, the "box" of space that surrounds the object. Consider it like it's frame. When you enlarge the object, the frame that contains it gets bigger. The boxes may sometimes be bigger than would appear to account for every part of an item. So it's possible that the bounding box for that object may have been creating the wall that you saw. "