Yes, the decision could be challenged as unlawful. Although whether it would be in anyone's interests to do so would depend on what would be the result of unwinding the matter and deciding it again.
If the clerk is still employed by the council, then the event could be raised as a factor in appraising the clerk's performance. Unless the consequences were very serious, the appropriate remedy would most likely be extra training for the clerk and a warning to be more careful in future. Disciplinary action might be seen as unreasonable unless the mistake could be shown to be deliberate and part of a wider pattern.
It's generally agreed that there should not be an agenda item "Any Other Business" because it encourages decisions to be taken without notice. Alternatives are "Matters arising [from minutes]" and "Questions to the Chairman". But these must still be used only to raise matters or keep them alive, not to take decisions without first alerting the public through the agenda.
The above are all views based on a limited understanding of the precise circumstances and should be treated as such!