The premium rate phones watchdog, Icstis, has ruled that Channel 4's Big Brother breached its guidelines during the series which finished in August.
Nikki ended up coming in 5th place despite already being evicted
More than 2,600 complaints were made after it was announced that contestants - already evicted by a phone vote - would be allowed another chance to win.
Viewers claimed they had been misled after paying to vote out housemates.
Channel 4 admitted the oversight. It escaped a fine but must pay almost £50,000 in administrative costs.
On reaching its decision, Icstis did accept that Big Brother was an "editorially inventive show".
But it added: "The unprecedented number of complaints for this type of service, combined with the undoubted strength of feeling of the complainants, clearly indicated that voters genuinely felt that they had been misled over this twist."
Viewers had been told they were voting to evict contestants from the Big Brother house permanently.
Grace, Lea, Mikey and Nikki were later re-introduced to the show to stay in an adjacent house.
Nikki was eventually chosen by existing housemates to return to the main house, giving her another chance to win the £100,000 prize.
Voters complained they would not have bothered ringing in to get a contestant evicted if they had known the rules of the competition would change.
Susie Verrico, 43, was the Golden Ticket winner
Earlier this week the "golden ticket" contest to find a Big Brother housemate was criticised by the Advertising Standards Authority.
The watchdog said Nestle and Channel 4 should have been clearer on terms and conditions on marketing material for the contest on Kit Kat bars this summer.
Nestle said it did not mean to mislead and promised to learn from the ruling.
The search for an extra contestant on the popular reality show saw 100 special tickets hidden inside Kit Kat chocolate bars.
But some adverts promoting the "golden tickets" failed to mention that entrants had to be aged 18 or above, while a poster failed to include the competition closing date.