Overlapping between Mapnicks data-snapshot and your edits. Wait a few days and everything will look well.
If you want to check if the data is correct, turn on the data-layer (blue “+”-Icon upper right corner).
The map is made up of a series of tiles which are drawn by the render engine. It looks like the tile on the left has been redrawn with the new changes but the tile on the right hasn’t yet been redrawn. I’ve had a case where mapnik took the almost a week to redraw a particular tile. Osmarender seems to update much quicker. Mapnik will eventually catch up but if I want to see more up to date images of the map then I tend use osmarender instead.