I’ve the following problem: in the last couple of days I mapped a small town in South-Croatia (http://www.openstreetmap.org/?lat=43.00699&lon=17.27424&zoom=16&layers=B000FTF). In the course of this, I also updated the coastline. The roads and ways are updated after a while (hour(s)?), but NOT the coastline (it’s already 7 days old).
Am I doing something wrong, or is this a bug in OSM?
For the Mapnik layer coastline is not rendered directly from live OSM data but it is preprocessed to static sea polygons first. This happens only now and then and therefore it will take some time before you can see your changes on the main map.