Some years ago I scratched a tiny horizontal line on my Brompton seat post so I could find the right saddle height easily. I figured since it is steel there would be no problem. Riding home yesterday I felt the saddle slipping away beneath me and discovered the post had broken (torn) perfectly evenly. The steel seat post walls are very thin and I suppose over the years that weakness just allowed the metal to fail. Of course, I can only blame myself for this and a new post is already ordered and on its way. Still, if anyone else is tempted to mark their post with anything that affects the metal it would probably be best to avoid making my mistake..