I was pretty sure I was missing at least one hat from years ago. I even made a thread about it. But unfortunately the only evidence I had was that I was really, really sure that I bought it. There was also a head and some furniture I could have sworn I had purchased. But I was told there's no reason for an item to just "disappear". I don't know about that, but since this was from over 2 years ago, there's just no way I can be 100% certain.
I have a feeling there are rare instances in which a hat or other item has disappeared from someone's inventory due to a freak glitch or bug (or perhaps even admin error?), but I don't think much can be done about it, unless there's solid proof that the player had said hat/item.