When I worked on a commercial HTTP proxy in the early 2000s, it was very common for servers to return off-by-one values for Content-Length - so much so that we

When I worked on a commercial HTTP proxy in the early 2000s, it was very common for servers to return off-by-one values for Content-Length - so much so that we had to implement heuristics to ignore and fix such errors. It may be better now but a huge number of libraries and frameworks would either include the terminating NULL byte in the count but not send it, or not include the terminator in the count but include it in the stream.