Depending on the server technology used, sometimes the map extent reported by raster layers may be smaller than the actual area which can be rendered (especially notably for WMS servers with symbology which takes more space than the data extent). Previous version of QGIS would crop raster layers to the reported extents, resulting in truncated symbols on the borders of these layers. Now, there's a new option to allow you to override this behaviour and ignore the reported extent for affected servers.
This feature was developed by Matthias Kuhn (Opengis.ch)