Baffling Scroll

A sheet of paper/parchment. clay tablet, etc suitable as a vehicle for writing a spell on. It operates as a standard 'scroll', but after a spell/rune has been inscribed - the next time it is looked at the spell may have changed.

d6
1-2  no change, the spell sticks
3    the spell is reversed if applicable, or sticks
4-5  the spell is replaced with random spell of same 
       level/school/realm/etc (cannot be the same spell)
6    the spell is gone, the scroll can be inscribed again