Server: Smith & Forge (private server), but has been observed in the wild elsewhere
Description: Sometimes, stacks of items are actually in your inventory but do not appear to be there. This results in effectively being able to carry less for the same encumbrance stat point investment. There appears to be a correlation between the incidence of this “disappearing item” issue and dragging into your inventory (vs. harvesting), but there’s anecdotal evidence that inventory-crafted items can disappear too.
While it has been widely observed on both official and private servers, it seems to be more common on private servers with increased crafting rates (i.e. faster crafting).
Workarounds: Logging out and log back in generally fixes it. The location of items in your inventory is refreshed and you can find them.