The ep_pair example demonstrates the endpoint double-buffering feature in our part. Since EP2IN and EP2OUT are both double-buffered, you should see a
pending OUT (in the "Pending Ops" window from the EZ-USB Control Panel) transfer after four consecutive bulk transfers in the OUT direction. At that point, you'll need to perform consecutive bulk IN transfers to empty the buffers and see no transfers pending.
Similarly, you'll see a pending transfer for an IN if previously you had not performed a bulk OUT transfer. This is because there is no data in the OUT endpoint yet. Please note that the EZ-USB FX is obsolete.