Saints appeared to be coasting towards a comfortable win until an explosive final 10 minutes, with the referee Nigel Miller at the centre of the action.
The visitors had started confidently - Ricardo Fuller and Kamil Kosowski exchanged passes and when the cross was whipped in, Matt Oakley must have thought he had scored but Lee Camp reacted superbly to keep the ball out with his left leg.
Derby hit back with a 20-yard free-kick from Inigo Idiakez which was heading for the top corner until Darren Powell headed clear with his keeper wrong-footed.
That was the best of the first-half action, which gave little hint of the drama that was to unfold.
Five minutes after the break Dennis Wise struck a post from a free-kick, before Southampton broke the deadlock in the 58th minute.
Fuller flicked a Wise free-kick into the path of Brett Ormerod, who cut inside to give himself plenty of time to side-foot the ball beyond Camp from six yards.
The Derby keeper then kept his side in the game when he reacted smartly after Fuller had connected to a near post corner from Wise.
On 80 minutes Derby were thrown an unexpected life line when Danny Higginbotham was ruled to have handled inside the 18-yard area.
Antti Niemi dived to his left to beat away Idiakez's first attempt but the referee ruled the keeper moved before the kick was taken.
There were heated protests from the Southampton players, who claimed Idiakez should have been penalised for checking his run up to the penalty.
The Spaniard kept his cool to roll his second attempt straight down the middle to level the scores.
Redknapp was none too happy with proceedings and when he made his feelings known to the fourth official, the referee had no hesitation in banishing the Saints manager to the stands.
Worse was to follow for the visitors in the 85th minute when Idiakez aimed a free-kick towards the six- yard area and Andrew Davies climbed higher than everyone else to score with thumping header.
The game had been stood on its head as far as Southampton were concerned and there was further misery in the closing stages when Tomasz Hajto was sent off after picking up his second caution.
But there was one more when, in the last minute, Niemi booted a long clearance towards the Derby goal.
Camp dashed from his area to head clear and was bundled down to the ground by Wise. The ball fell to Fuller 35 yards out and he kept his cool to lift the ball over the stranded keeper and into an unguarded net.