The 1880 Derby by-election was held when the incumbent Liberal MP Samuel Plimsoll resigned to find a seat for the recently unseated Home Secretary William Vernon Harcourt.
The Liberals had formed a government after winning the 1880 general election and Harcourt, a well known radical and talented debater, had been offered the job of Home Secretary. Under the law at the time he had to resign his seat and seek re-election. In a close fought by-election Harcourt lost his Oxford seat.
Samuel Plimsoll immediately resigned and Harcourt was unopposed in the subsequent by-election.
References
Derby by-election, 1880 Wikipedia(Text) CC BY-SA