Two were actual bugs, though they existed only in unused code: * In Freestyle it was unintentionally copying a scene rather than referencing it. * In BLI_array_store_is_valid there was use of uninitialized memory.
Problem reports independently by Light BWK and Folkert de Vries, thanks.
This fix should be considered for inclusion in the 2.73 release, since it concerns a new feature of Freestyle introduced in 2.73.