Most of the posts here are imported from sites that allow NSFW content, but only the ones that are marked as safe. This is why some content gets through and the moderators have to fix the problem. The tags are usually not changed, so you get stuff like BDSM and the like.
But seriously, the "Corrugated Galvanised Iron Sheet" tag is one of the most curious tags I have encountered. XD
Well that's one I'll have to check out.
Edit: you knew I was going to go straight to this one didn't you?http://safebooru.org/index.php?page=post&s=view&id=942809