Rangers led twice at The Den through Niko Kranjcar and Charlie Austin, either side of a Scott McDonald equaliser.
But Jermaine Easter's 93rd-minute strike secured a 2-2 draw and a share of the spoils for the hosts.
Redknapp told the club website: "It was two points dropped.
"To concede a goal with almost the last kick of the game is really poor from our point of view.
"You can't concede goals like that in the last minute at this level - it's really disappointing. We threw it away really."