This item has attribute 815 (use_model_cache_icon). Attribute 815 was likely intended for weapon skins, and is not supposed to be on this item.

When an item is Festivized, it uses a different icon for rendering the Christmas lights on it. With the first batch of Festivizable items, applying a Festivizer added attribute 815 to the item, which permanently changes how it looks. If the Festivizer is removed, the icon still uses the Festivized render, which it's not supposed to. This was fixed in early 2017, so the bug was out there for a few months.

Any item that had a Festivizer applied before the fix permanently has the attribute. Even unfestivizing such items now still keeps it.

It is very obscure. Usually only the item owner sees it in their backpack (and possibly when inspecting someone who has the item equipped). If the Festivizer was never removed, the owner would never notice, and even then it may go unnoticed unless there is another copy of the item in the backpack that renders differently. Interestingly, this glitch doesn't happen at all on Linux.

Some items, like the Kukri, show barely perceptible differences, while others are very noticeable: the Scottish Resistance shows at a different angle entirely, the Quick-Fix renders with a completely different color, and the Natascha looks different as well. The most dramatic one is the Spy-cicle, which disappears entirely, leaving just a blank tile in the backpack.