Source may have been the first engine to formally support lag compensation on the server side, but something similar was already implemented previously as a mod

Source may have been the first engine to formally support lag compensation on the server side, but something similar was already implemented previously as a mod for Quake 3 called "Unlagged". Essentially what it did was maintain a buffer of all player positions over the last X number of milliseconds and whenever a raycast check needed to be done, it looked back in that buffer based on the current ping of the player requesting the raycast. There is still some documentation left on the internet: https://www.ra.is/unlagged/faq.html https://www.ra.is/unlagged/faq.html https://openarena.fandom.com/wiki/ModCompat/Unlagged https://openarena.fandom.com/wiki/ModCompat/Unlagged