The wagon shows things that have been in it but are no longer there, sometimes doesn't show things that are, and all in all - behaves strangely. This seems to be related to how the items got inside the wagon (player putting them, inserter putting them, player Control+clicking, etc...)
Just to make it clear - I have nothing set in the filters, just an empty wagon showing things that used to be there, or doesn't show things that are.
(Edit: Just noticed typo in the title
![Embarrassed :oops:](./images/smilies/icon_redface.gif)