Honestly. This is ridiculous. It's been over a whole year, and yet they still haven't fixed the borderline broken item placement? I love this game, but how hard can it be to have your items stay where you place them? To not have weapons fly through display cases? I planned to decorate my Hearthfire house. But I can't. Know why? Because weapons and items decide to not stay where they're placed and go through my display cases, and because you are always forced to grab items from the bottom, and items seem to be magnetized to the specific side you do not want them to be facing. Oblivion's system was good enough. Oblivion's items didn't fly around when you came back. What happened? What went wrong? Why is this so difficult to do? Words cannot describe the amount of raeg this ridiculousness has caused me.

That's an engine limitation. Don't count on it being fixed until TES VI, if even then.

Not sure how well this works but... at the bottom of http://www.uesp.net/wiki/Skyrim:Houses page, the first bug issue describes a work-around.