Index: /trunk/data/validator/geometry.mapcss
===================================================================
--- /trunk/data/validator/geometry.mapcss	(revision 10388)
+++ /trunk/data/validator/geometry.mapcss	(revision 10389)
@@ -273,2 +273,8 @@
   throwWarning: tr("Waterway ends without a connection to another waterway or the direction of the waterway is wrong.");
 }
+
+/* #10717 */
+way[natural=coastline] ∈ area[natural=water],
+way[natural=coastline] ∈ area[waterway=riverbank] {
+  throwError: tr("Coastline inside {1}", "{0.tag}");
+}