OK, so two things:
1) Place it sideways to start with; if you can somehow make it "fall" from that position, you don't deserve nice things.
2) An X-box doesn't just spontaneously fall any more than I spontaneously combust. There was a reason for it tipping.
It might be an unstable platform being bumped by the family pet, or it might be you shotgun kicking it down the stairs; I don't know, I'm not an expert. But I can tell you what didn't make it fall: Microsoft.
EDIT: Somebody damn well better get the reference I'm making there.
Also, if it costs you 20 pounds to get it refurbished at the store, you're being ripped off. It only cost me like $4 to get mine refurbished.