See P8914 for a demonstration.
I make a fairly standard API request expecting a JSON response. Most of the time, it works fine. Increasingly (about 20% of the time?), I'm getting back the HTML for the main API help page without any indication that I did anything wrong.
I've only been able to reproduce this with POSTs. Is POSTing incorrect? I've been doing it this way for years, so that would be surprising, especially since it only fails sometimes.
Alternatively, I can add logic to my bot to retry in this case, except it seems more like a symptom that I'm doing something wrong rather than a server error—but I can't figure out what it is!