I am not sure if its fit in the "bad design" category you look for but OpenStreetMap API has various usability problems. https://wiki.openstreetmap.org/wiki/AP

I am not sure if its fit in the "bad design" category you look for but OpenStreetMap API has various usability problems. https://wiki.openstreetmap.org/wiki/API_v0.6 https://wiki.openstreetmap.org/wiki/API_v0.6 The main issue has I see it is that all API functions accepts only XML as content-type. There is no way to use JSON, which is rather standard by now. Even GET serves only XML. Another issue is that the error messages for non-conform requests are not informative enough. This is emphasize in the doc ("unfortunately error messages are not consistent"). Finally, in my opinion, the doc do not provide simple and clear overview of the API.