The Bandos crozier, although being a magic staff, was actually a one-handed weapon, allowing the usage of an item in the off-hand slot. This has now been fixed.
Prior to 24 March2014 this weapon was a popular choice for low level pkers. As it's only requirement to wield was 60 prayer it allowed even level 4 accounts to wield croziers while having the accuracy of a level 50 weapon. The update however changed the crozier to be wieldable item, with only a prayer bonus (increased from +3 to +6). The crozier is also no longer classified as a weapon.