Tidying up some data today: here are eleven particularly shady (or particularly unlucky) MPs who managed to have their elections overturned on petition *twice*. https://w.wiki/kep 

A particular salute to George Whalley, John Harris, and Andrew O'Dwyer, who managed it twice in the same year. That's dedication. Harris and O'Dwyer took the hint and retired; Whalley got in again a few years later, and then went completely off the rails.

Number of successful election petitions by parliamentary term, 1859 onwards (includes by-elections as well as general elections) https://w.wiki/kes 

The numbers are even more dramatic when you take into account that large numbers of seats were still uncontested at this point - and presumably petitions were only ever brought to challenge a contested election.

Looking at just the general election petitions ( https://w.wiki/ket ) & comparing to # of contested seats:
* 1859 - 10 out of 275 (3.6%)
* 1865 - 15 out of 355 (4.2%)
* 1868 - 23 out of 446 (5.1%)
* 1874 - 15 out of 465 (3.2%)
* 1880 - 17 out of 543 (3.1%)

And this is just the *successful* petitions! There were many more that failed, or were withdrawn. Assuming the list on WP is complete, there were 52 election petitions brought in 1868. Almost one in eight contested returns were challenged - though some on quite shaky grounds.

They make remarkable reading - on the one hand, we have the election in Beverley where 800-1000 people were bribed (more than half the voters!) https://en.wikipedia.org/wiki/List_of_UK_Parliamentary_election_petitions

On the other, we have Northallerton, where "the treating alleged amounted only to a suspicion about a single glass of beer and a cigar, and the intimidation amounted only to a squabble between two people."

In Penryn and Falmouth, the winning candidate was accused (apparently spuriously) of having tried to bribe a voter by promising a cure for deafness if he voted for him, which is definitely a creative manifesto commitment.