The www says that a ‘code de status:509’ warning is ‘use of too much bandwidth’, Looking at the background suppose this is with ID Editor with which I’ve never encountered this, ID fetches background data on-the-fly, but as with the JOSM editor, if selecting too large an area and then trying to fetch that in 1 go you get a bad request warning or with a very large relation it starts fetching the members and then times out at some point.
Since the OSM servers don’t have infinite bandwidth we at times do see messages like ‘too many users connected’ preventing upload or download of data so if you did mass downloads you are affecting other users as well, hence can see why this was implemented by the OSM server room operators.