Some of the trickiness that comes from HFT and adjacent things like this is you can be working on tremendously powerful hardware but still be miserly about bits because every extra byte you have to cram into a packet is extra latency. The HFT firm I worked for would "re-use flags" in the sense that each packet literally had an 8-bit section called "flags" (and further down in the message, another 8 bit section called flags2 because of course) and each bit in there was a Boolean that could be on or off - a flag. So we weren't reusing flags as much as we were re-allocating what a high bit at that index in flags meant.

We were very conscious of this kind of error though and we managed them like Scrooge counting his farthings.