Sheffield City Council is considering fining a contractor which wrongly sent demands for housing benefit overpayments to hundreds of pensioners.
Urgent meetings to discuss the administrative "cock-up" have taken place between council leaders and contractors Liberata.
The authority said it was outraged by Liberata's mistake, which led to 785 residents receiving bills for benefits.
Liberata has apologised and made a £1,000 donation to Age Concern.
The council will carry out an investigation before making a decision on a fine.
Assistant head of finance for the authority, Andy Dolling, who earlier described the blunder as a "cock-up", said any financial penalty imposed on Liberata would be put back into council services for the elderly.
Deputy leader of Sheffield City Council, Councillor Steve Jones, said: "This situation is totally unacceptable and we have made sure that Liberata understand the seriousness of their mistake.
"Any further incidents of this nature will have very serious repercussions for the partnership arrangement between the council and Liberata."
Liberata Chief executive Robert Gogel said it had taken steps to make sure the "unintentional administration error" did not happen again.
"As soon as we discovered it we apologised by letter to the individuals concerned." he added.
Liberata, which handles a number of council services including benefits and council tax, has also donated £1,000 to Age Concern as a goodwill gesture.