I dropped in your better_nested_set plugin to a class I had used with the standard acts_as_nested_set, and began seeing failures on tests that passed previously. My model declares non-default names for :left_column (:lvn) and :right_column (:rvn), and after dropping in better_nested_set, these attributes started returning nil instead of the actual left and right values in my model.
Playing around with this, I found that although my attributes were still available (and contained the correct values) in the attributes hash, accessing these attributes using accessors returned nil.
I have managed to get this working by commenting out line 109 in better_nested_set.rb, which defines accessors for the attributes used for :left_column and :right_column.
I'm not sure why these are necessary, and this seems to solve my immediate problem... not sure if it will cause others!
