Recent changes on the main rendering servers now mean that the software used to construct polygons & ways (osm2pgsql) no longer tries to fix problems such as the self-intersection in the outline of this lake. This is just one example of many polygons (and, less likely, linear features) which will no longer be rendered on the Standard Carto-OSM layer. In the short term this will mean things disappearing from the map, but in the longer term this is expected to result in much better quality data for data consumers because issues will be fixed at source in OSM.
Christoph Hormann (Imagico) has written a good description of the changes and their rationale here: [https://www.openstreetmap.org/user/imagico/diary/44711][1]
As @nevv @nevw says in their answer, use of data quality tools such as OSM Inspector, Osmose etc., can rapidly identify these problems. I wrote a [blog post][2] long ago about using the first of these for this purpose.
[1]: https://www.openstreetmap.org/user/imagico/diary/44711
[2]: http://sk53-osm.blogspot.com/2011/02/exploration-of-bad-polygons.html
