Two bin lorries and five vans owned by South Norfolk Council remained on the road despite not having a valid MOT.
The MOTs on the seven vehicles expired on May 31 and continued to be used on the road throughout the summer.
Council officials have blamed a typing error for the oversight, with one of the vehicles involved in an accident while it had no cover.
The incident, which involved a Sprinter van used by the council and a car, occurred in July and resulted in parties involved requiring hospital treatment.
A member of staff has been suspended while council chiefs probe how the mistake was not picked up, reports the Eastern Daily Press.
South Norfolk Council said: “Our initial enquiries have shown a staff member incorrectly typed 2019 and not 2018 into the maintenance database as the renewal date for a small number of vehicles.
“Seven out of our fleet of 55 vehicles including light vans were affected and all of the refuse vehicles have passed their regular six-weekly safety inspections.
“As soon as we were made aware of the situation the vehicles were taken off the road and we immediately arranged for those vehicles to be booked in for MOTs.”
In the meantime, the council is to launch a full investigation into the incident and how it was not noticed sooner.
A council spokesman added: “A staff member has been suspended pending a full investigation.
“Our Ketteringham depot has been in operation for 40 years serving the residents of South Norfolk and this unfortunate administrative error is not a reflection on the hard work and dedication of our bin crews.”
The spokesman added that the DVLA and the Department for Transport had been made aware.
All seven vehicles are awaiting new MOT inspections and will remain off the road until these have been carried out.