Biggest mistake the Germans made in Stalingrad was bombing the city to rubble, the rubble made it easier for the Russians to defend.
The biggest mistake the Germans made at Stalingrad, was even being in Stalingrad. There was literally no strategic reason to seize the city. It was a pure ego play to take the city with Stalin’s name. A modern military would have simply bypassed it and seized the nearby oil fields. The forces in Stalingrad did not have the capability to strike out at such a move. A land-based version of the island hopping strategy was strongly called for there. For all the talk of German prowess, they made some astonishingly bad plans. And they didn’t all come from Hitler. Another horrifics decision the Germans thankfully made was splitting the force into 3 giant army groups on three Avenues of advance. And I think the reason they did that is that they believe their own propaganda that the USSR would simply implode as soon as they crossed the border. That old quote about the whole rotten house collapsing. German plan was never a serious plan that anticipated resistance. They got overconfident after a string of victories over small unprepared countries.
That was a big one but there was an even bigger one:
The drive to Stalingrad was executed by an German general extremely proficient in the field. When he went sick Hitler replaced him with a General who was a GENIUS at HQ on the telephone and on paper but was verrry thin on field expertise.
The soon-to-be defenders of Stalingrad were on the run, nearly totally encircled and doomed when this inexperienced General happened on the pre-Stalingrad scene.
"OK, taking the city is going to be a biiiig job, so rest up..!"
For a casual hiker or camper it sounds pretty ordinary common-sense but it was an unthinkable blunder, the very worst type of thing an experienced General would ever do:
Which would you prefer to fight; a war of German mobility using your tank forte against scads of fleeing, on-the-run Russian troops, without fighting positions, or instead fight a war of little mobility in a city teeming with dug-in defenders utterly contemptuous of death..?
The first route favors the German Army, the second has been the preserve of Soviets for over 100 years.
He should have pressed his advantage at any cost immediately, encircling the harried Red Army on the open expanses of the steppes, then taking the mostly undefended Capital city.
Stalingrad was lost in the 3 or so days right before the actual Battle of Stalingrad.