Leading by 46 points at the final change, Melbourne lost following a remarkable Nasiah Wanganeen-Milera goal after the siren to give the Saints the biggest three-quarter-time comeback in AFL history.
A post from an anonymous account on X, formerly Twitter, implied they were waiting near Goodwin's car ready to harm him.
The Demons are aware of the post and have reported it to the AFL integrity unit, declining to comment further.
The post surfaced during Goodwin's press conference, where the shattered 2021 premiership coach was trying to piece together how Melbourne blew their seventh win of the season.
"Sometimes in your darkest moments can be your biggest growth," Goodwin said.
"When you have a last quarter like that, it's a pretty devastated group in there.
"It's a pretty devastated group of coaches, because we're here to win."
Wanganeen-Milera calmly slotted his third goal after flying for a soaring mark with less than 20 seconds remaining.
That levelled the scores, before a 6-6-6 penalty in the middle was paid against the Demons, to the confusion of all players.
Saints ruck Rowan Marshall was able to perfectly pick out Wanganeen-Milera running inside 50 to take a mark just before the siren sounded.
Any score would have given the Saints victory, but Wanganeen-Milera went back and kicked the goal.
"Clearly, we didn't get that right," Goodwin said.
"It's a pretty simple process to get 6-6-6 done.
"You need to communicate, get organised.
"We didn't get that done ... that's on us to get that done the right way.
"We'll have a look at why it happened, how it happened, and make sure that it never happens again."
St Kilda kicked nine unanswered goals in the last quarter, but a calamity of errors from Melbourne helped them pinch the win.
The Saints had the lead for a little more than a minute of the entire game after slotting the first goal of the match.
Experienced midfielder Clayton Oliver had a chance to almost ice the game when he had a shot on the run with the Demons still in front by six points, but he sprayed the kick completely and failed to score.
Melbourne superstar Kysaiah Pickett also got stranded on the bench in the dying stages, only returning with eight seconds left.
"When you've had seven or eight goals kicked against you in a row, you need to find a way to execute those things under pressure, and we didn't do that," Goodwin said.
"We'll learn from it clearly and grow from it, but it's unacceptable at the same time."
In a bizarre season, Melbourne went 0-5, then won five of their next six games to put themselves back in the finals picture.
But the Demons have lost seven of their past eight matches, the only victory in that time coming against lowly North Melbourne.
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