g_closure_new_simple has the annotation (transfer full) for the return value but returns a floating GClosure.  Generally, functions returning floating objects/structs are annotated with (transfer none) so that the caller will perform a ref+sink.

As it stands, with (transfer full), a caller would just assume the existing ref, but later on, as the floating flag is still set, a ref+sink could end up assuming ownership of the same reference.

Note that g_cclosure_new_swap (defined in terms of g_closure_new_simple) probably needs (transfer none) for similar reasons, though this function is annotated with (skip).