Ok, that title is a little weird, so let me explain:
If you have a BoE weapon that can be equipped to either hand (say, swords on a warrior), and you attempt to equip it to your off hand before binding it, it will equip to the main hand instead after the dialogue box. If A, B, and C are all green weapons…
A equipped to Main Hand
B to Off Hand
C unbound.
Right-click C, select equip off-hand
binding dialogue box (accept)
end up with C and B equipped instead of A and C