a possible source is in the replies to that tweet: https://twitter.com/RobbieBC/status/419324772754132992

Honestly it wouldn't surprise me if it had been invented multiple times independently, it's just too obvious to anyone that has had to deal with calculation.

(But thanks for sharing what us without a Twitter account cannot see otherwise anymore…)

Clearly we must now start referring to it as ‘Carmack’s sign error joke’ ;)

had you not mentioned that, we might have gotten another fast inverse square root origin investigation

fast (multiplicative) inverse square root.

autocorrect or dyslexia. whoever wins, i lose