The Panther's thick front armour could not be penetrated by any Soviet tank gun in use in 1943. This immunity, however, did not apply to the side armour, which turned out to be vulnerable to even the rapidly ageing 45 mm gun.
The sloped part of the sides proved a tough nut to crack. The 45 mm gun doesn't work here, and hits from 300 and then 100 meters do not penetrated. However, other parts of the tank can still be penetrated from this side: the turret has two holes in it from 45 mm APCR (300 and 400 meters) and one from 45 mm AP (400 meters). The vertical side of the hull (unfortunately not pictured) can be penetrated from 500 meters. The rear was also penetrated from 300 meters (also not pictured).
The 76 mm gun has no issues with this armour at all, however. Even firing at an angle of 60 degrees, from 600 meters the armour piercing shell penetrated the side armour. The sloped hull did poorly against this weapon, as you can see, forming a fairly sizeable breach. Despite the thick front plate, even an ordinary T-34 would not have to flank very far to be able to destroy its much heavier opponent.
CAMD RF 38-11469-40 p.22