When implementing a GstBaseTransform where:
* Incoming buffers are writable (refcount == 1),
* AND we don't implement a prepare_output_buffer() method in the subclass
* AND The src and sink caps are the same

The buffer passed as argument to the transform_ip() virtual method has one reference too much, .... which makes the buffer un-writable (refcount > 1) !

The responsible part seems to be here (gstbasetransform.c:889):
        /* Input buffer is already writable and caps are the same, just ref and return it */
        *out_buf = in_buf;
        gst_buffer_ref (in_buf);