Notices of intended prosecution claimed 991 drivers had broken a 40mph (64km/h) speed limit in Conwy tunnel last October.
But the limit had been imposed for night maintenance only and not lifted in the morning as it should have been.
Within days the drivers got an apology in the post.
North Wales Police released the figures in a Freedom of Information reply.
The force said: "The issue was caused by an administration error surrounding the enforcement period.
"North Wales police do not record the cost of cancelling notices."

The suspended Uefa president, 60, was banned by the world governing body's ethics committee in December, along with its president Sepp Blatter.
Both were found guilty of breaches surrounding a £1.3m ($2m) "disloyal payment" to Platini.
Blatter's appeal against his own ban is due to be heard on Tuesday.
The former France international and the Swiss have both denied any wrongdoing.
Platini was also fined £54,000 ($80,000) after being found guilty of breaching Fifa's code of ethics.
Both Platini and Blatter said the payment was made following a verbal agreement between the pair when the Frenchman worked for Blatter from 1998 to 2002, though it was not paid until nine years later.
The Fifa appeals committee, which is chaired by president of the Bermuda Football Association Larry Mussenden, has the power to reduce, increase or overturn the ban.
The ethics committee's investigatory chamber has confirmed it is appealing against the sanctions imposed on Platini and Blatter on the grounds that they are too lenient.
Uefa said last month that it will not hold an election for its presidency until Platini's appeals process has been concluded.
Platini had at one stage been favourite to succeed Blatter as Fifa president, but the opening of ethics proceedings against him and subsequent ban ended any chance of that happening.
Fifa's presidential election is due to take place on 26 February.
On Friday, the committee banned the governing body's former secretary general Jerome Valcke for 12 years for multiple breaches of the code of ethics.

His comments come as the Tibetan exile movement meets at its headquarters in the northern Indian town of Dharamsala to discuss its future strategy.
There is growing concern over the number of Tibetans self-immolating or setting themselves on fire.
Activists say 51 Tibetans have set themselves on fire since 2009.
The Tibet issue: Two views
Inside Tibet
The Dalai Lama and the Tibetan government-in-exile are under increasing pressure - not just from Beijing but now also from their followers who are divided over how to continue their struggle against Chinese rule.
Some of those gathering in Dharamsala this week want a more assertive policy, including more open backing of Tibetans setting themselves on fire, while others say this tactic is against Buddhist teachings and want another approach.
The Dalai lama has steered a middle course.
In a BBC interview, he said the immolations were a symptom of China's repressive rule in Tibet and called on its new leadership to accept the need for change.
"The new leadership must use common sense and a more holistic view to serve long term-interest. There is no other way.
"Just using force, censorship and to remain a closed society is almost like suicide. Judging that way, I feel there is possibility or a real chance to change."
Beijing says Tibet is an integral part of China and accuses the Dalai Lama of trying to break the Himalayan region away.
The Dalai Lama says he only wants greater autonomy.
But, he said, it was getting harder to reach an agreement because of an increasing divide between ethnic Tibetans and the Han Chinese who have been encouraged to move there by Beijing.

The man was hit by a Brighton to London express train travelling at 80mph on 24 June 2014.
He was leading a team of 12 repairing cracks in the track when he was stuck on the right shoulder and thrown down an embankment near Redhill, Surrey.
Network Rail pleaded guilty to two breaches of health and safety law.
Rail inspector Tom Wake said: "Network Rail's planning of track maintenance work fell below legal standards, placing workers in unnecessary danger and causing an employee to suffer life-changing injuries."
Network Rail said it had since banned working while trains are running on that stretch of railway.
Its south east managing director, John Halsall, said: "Safety is our absolute priority and it is clear that we fell short in this instance."
Nearly 1,000 drivers were wrongly sent speeding notices after a temporary limit on a north Wales road was not lifted, figures have shown.
[NEXT_CONCEPT]
Michel Platini's appeal against his eight-year ban from all football-related activity will be heard by Fifa's appeals committee on Monday.
[NEXT_CONCEPT]
The Tibetan spiritual leader, the Dalai Lama, says he hopes for a new Chinese approach to Tibet with the expected change in its leadership next month.
[NEXT_CONCEPT]
Network Rail has been fined £800,000 for an accident which left a track worker with multiple serious injuries.