The U.S. maintained that the attack was a "mistake" but it defies logic. The government troops had been there for some time and minimal radio traffic analysis would have clearly shown that those troops were communicating on SAA tactical frequencies, not to mention multiple opportunities to visually inspect the positions by satellite or photo.
More to my point here, the Russian liaison officer attempted to contact his U.S. counterpart over some sort of hotline to call off the strike on the government troops but the latter officer had mysteriously gone "walkabout" and was unavailable. The Coalition strike lasted for, or continued for, about an hour with multiple SAA deaths and injuries. The American position that an officer standing watch at one end of a hot line could wander off for a coffee break with no one to cover for him in his absence is absurd.
Having established itself as an entity given to lies, the U.S. took the position that its attack on a Syrian air force fighter-bomber was necessary because it was attacking Syrian Democratic Force (SDF) troops, or too close too them, is worthless.
In the case of the attack on the Syrian jet, it isn't that a watch officer was absent from his post but that the U.S. did not make any attempt even to use the available coordination channel to avoid tragic errors.
The [Defense Ministry of the Russian Federation] explained the reasons for suspending the memorandum [for preventing incidents in Syria's skies]. According to the ministry, at the moment the Syrian warplane was hit by the US fighter, Russia’s Aerospace Forces were carrying out missions in Syria’s airspace; [sic]Apparently, no warning was given by the U.S. plane to the Syrian pilot either. He was just blown out of the sky. Other commentary on the web has also made the point that Turkey has attacked SDF troops with no retaliation or defensive response by the U.S.
"However, the coalition command did not use the existing communication line between the air commands of Al Udeid Air Base (Qatar) and Khmeimim Air Base to prevent incidents in Syria’s airspace."
"We consider the actions of the US command as a deliberate default on their obligations under the memorandum on on [sic] preventing incidents and providing for safe flights during operations in Syria signed on October 20, 2015," the Defense Ministry stressed.
As with the attack on Deir ez-Zor, this June 19, 2017 attack on a Syrian war plane smells.
 "Russia cuts deconfliction channel with Washington after US downs Syrian jet." By TASS, 6/19/17 (emphasis added).