The Authoritative Reference on Eisenhower dollars Second Edition states that it did happen. By John Wexler, Bill Crawford and Kevin Flynn. They call them transitional planchet errors.
They were struck in 1974, 1976 and 1977 at the Denver mint. Rejected copper-nickel proof planchets were sent to Philadelphia or Denver to be used as business strikes. Somehow some of the rejected silver-clad planchets got mixed in with the rejected copper-nickel planchets and sent to Denver. This happened for each of these years.
Since there were no silver proofs in 1977 they think they were left over from 1976. They estimate 10-30 for each of these years.
They were struck in 1974, 1976 and 1977 at the Denver mint. Rejected copper-nickel proof planchets were sent to Philadelphia or Denver to be used as business strikes. Somehow some of the rejected silver-clad planchets got mixed in with the rejected copper-nickel planchets and sent to Denver. This happened for each of these years.
Since there were no silver proofs in 1977 they think they were left over from 1976. They estimate 10-30 for each of these years.


















