Admittedly I’m inexperienced here (and so may well be missing something), but in T354858, I received 429 errors that do actually seem to be as a result of a technical problem (rather than the same client repeatedly making requests).

@A_smart_kitten usually what happens is that the first few users get a HTTP 500, then the throttling logic detects that users are repeatedly requesting thumbnailing (instead of fetching a thumbnail from Swift) and starts blocking those requests. So it's true that for thumbnails 429 usually means there's an error, but that specific request doesn't trigger the error, and so its details are not particularly useful for debugging.