The Several Million Dollar Bug
Some IPSes (Intrusion Protection Systems) that perform deep-packet inspection won't pass such traffic.But this isn't really a "bug" per se; the TCP model is a stream is a stream is a stream.  There's no notion of time, packets, or correlation between streams.  So browsers (and the OS) are acting only as they can; by treating a TCP connection as two independent streams.(Though, how could it be otherwise?  Assume HTTP over SCTP (sequenced packets).  We can't require, or even allow, HTTP servers to ignore response packets that arrive "too early", since it's possible that observers of the client (e.g. Wireshark) may not observe the exact same timing, which would lead to divergent interpretations of the conversation.)Amazon does this too.  Upload APIs will return 4xx errors well before the body is uploaded in the event there's an issue with the headers.  Not that (a) most HTTP clients pay attention to this, or (b) they can do anything about it without closing and reopening the connection.