The centre-back saw his close-range effort hit the angle of post and bar, bounce up and then strike the bar again on 88 minutes.
Set up by Adam Clayton, it summed up a frustrating afternoon for both sides in a tale of two halves.
Doncaster, unbeaten in six games against Town, should have been in front at the break after a commanding first-half display.
Former Huddersfield striker Theo Robinson set the early tone with a shot wide from distance after just 15 seconds.
The Wales cap crossed two minutes later for striker Chris Brown as the visitors had a goal ruled out for a foul on Smithies.
Brown, whose header was tipped by away by the Town academy product, forced Smithies to drop the ball and then prodded home after Cotterill put it back in.
Ex-Preston striker Brown was denied by James Vaughan's goal-line clearance after another great delivery from Cotterill on 14 minutes.
Cotterill created further chances for Brown and Richie Wellens and then twice went close himself.
He flicked a volley just over from James Coppinger's cross on 26 minutes and just before half-time he had shot blocked by Lynch.
Huddersfield, unbeaten at home since March, were hardly in a first half played in front of a strangely subdued Yorkshire derby crowd.
Their best first-half chance came on 38 minutes when Vaughan was denied a penalty after he latched onto a backpass from Rob Jones.
The Town striker nudged the ball past Ross Turnbull in the Rovers goal and then appeared to be felled only for referee Paul Tierney to wave play on.
Vaughan, who had netted eight times in six games, was earlier denied in a goalmouth scramble caused by Martin Paterson's cross.
Paterson, a scorer for Northern Ireland in mid-week, was replaced by fellow striker Jonathan Stead as boss Mark Robins sought to inject life into Town's display.
His introduction had the desired effect, with the ex-Sunderland hitman forcing Turnbull into a good save just moments after coming on.
He then jinked his way into the box on 60 minutes, drew Turnbull and squared for the unmarked Jonathan Hogg, who somehow blazed over.
Vaughan was denied by Turnbull's legs on the edge of the box five minutes later after Clayton and substitute Sean Scannell combined in a sweeping move.
Cotterill, less of a threat after the break, nearly set up a winner for Donny with a cross from the right which Brown headed inches wide on 82 minutes and though Lynch's effort nearly sealed victory for Town, both sides settled for a point.
- Sports & Recreation
- David Cotterill
- Adam Clayton
- Alex Smithies
- James Vaughan
- Ross Turnbull