@attributes_cache in AR model set to nil?

Hi,

did anyone ever ran into the problem that @attributes_cache being set
to nil somehow? That problems occurs running "rake spec" but not
running the single spec. The error occurs e.g. here:

http://github.com/rails/rails/tree/master/activerecord/lib/active_record/attribute_methods.rb#L305

Thanx a lot,

Tobi