Cameron Jerome’s 18th goal of the season gave the Canaries a deserved lead at the break and the hosts were controlling the game when disaster struck in the 66th minute.
Ruddy had had little to do all afternoon but when required to catch an inswinging corner from debutant Jamie Hanson, he somehow fumbled the ball into his own net to gift the defensive-minded Rams a point.
The clash between second-placed Derby and fifth-placed Norwich got off to a cagey start, with Bradley Johnson’s off-target shot from long range for the home side the only effort of note in the opening stages.
Norwich were very much on the front foot, though, and Alex Tettey and Jonny Howson also tried their luck from distance with no success against a Derby side who looked as though they had come for a point.
On-loan midfielder Graham Dorrans then had a clear sight of goal, only for a poor first touch to allow Johnny Russell to clear for a corner, before Cameron Jerome steered the ball just wide after being put in by Wes Hoolahan.
Without their main men up front – Darren Bent and Chris Martin – the Rams barely posed a threat at the other end and it was no surprise when Jerome opened the scoring for the hosts on 31 minutes. A slick one-touch move instigated by Tettey ended with Martin Olsson crossing low for Norwich’s top scorer to slot home from close range.
Derby’s appeals for offside were waved away and Norwich continued to dominate, although Russell’s quick dart into the box did provide a rare moment of alarm for the home side.
Norwich maintained their grip on the game in the early part of the second period, with Jerome heading over from a corner before Lee Grant’s parry from a Steven Whittaker snap-shot prompted a scramble in the visitors’ box.
Derby’s lack of a specialist frontman was painfully obvious, although Tom Ince’s low shot on the hour mark did draw a save out of Ruddy.
But the Norwich keeper then made a horrible mistake to hand the Rams an equaliser they scarcely deserved. Hanson’s 66th-minute corner from the right should have been a routine gather for the former England keeper but he took his eye off the ball and somehow fumbled it into the back of the net.
City responded well to the shock and it required a brilliant one-handed save from Grant to keep out Sebastien Bassong’s header from an inswinging Hoolahan free-kick.
Grant then denied substitute Gary Hooper as Norwich looked for a grandstand finish before Jerome wastefully headed a Whittaker cross just over and Johnson then followed suit with his left foot.