...it's small, but found a bug in the new filter code in the ActionController for rails edge.
Showed up using habtm with ActiveScaffold - go figure.

Lesson learned - Array.insert should probably be named Array.insert! - it changes the underlying array, and sometimes that's not what you mean to do.

http://dev.rubyonrails.org/ticket/8383

Now we'll see if it gets committed.