The transform function for a GstGLSyncMeta copies the GL sync object to the new meta, but there's no refcounting. Whichever of the GstGLSyncMeta's is freed first will invalidate that sync object within GL. The 2nd one will (if we're lucky) throw a warning, or (if we're not) invalidate someone else's sync object if the same pointer has been reused since.

Warning output:
0:00:02.140869766 31827 0x7fffd408dcf0 ERROR              glcontext gstglcontext.c:1078:_gst_gl_debug_callback:<glcontextglx0> high: GL error from API id:4, GL_INVALID_VALUE in glDeleteSync (not a valid sync object)

Guilty code:
  if (GST_META_TRANSFORM_IS_COPY (type)) {
    GstMetaTransformCopy *copy = data;

    if (!copy->region) {
      /* only copy if the complete data is copied as well */
      dmeta = gst_buffer_add_gl_sync_meta (smeta->context, dest);

      if (!dmeta)
        return FALSE;

      GST_LOG ("copy sync object %p from meta %p to %p", smeta->glsync,
          smeta, dmeta);

      dmeta->glsync = smeta->glsync;
  }