If the Woolas judgment is the first election result since 1911 to be overturned on the grounds of corrupt practice, then presumably the last one was the case of C.F.G. Masterman, whose victory at West Ham North in the December 1910 general election was overturned by an election court in June 1911.
In Masterman's case, the verdict turned on irregularities in the declaration of election expenses by his agent, including submitting a return over the permitted spending limit and failing to declare the cost of leaflets donated to the campaign, by the Free Trade Union and others. Remarkably, the counsel for the petitioners (i.e. the Conservative candidate was at pains to stress that 'no imputation was made on the honour, integrity, or conduct of Mr. Masterman in relation to such matters', a proposition endorsed by the judges.
However, Masterman appeared to have a bit of a problem in choosing election agents. He stated in the course of his evidence that he had had three different agents in his three contests in West Ham North. When he stood in 1903 at a by-election in Dulwich, he was also rather prickly in his dealings with Liberal party HQ over who he would and wouldn't have as agent. In general a rather highly-strung character, he might well have been difficult to work for.
For him, this triggered a series of events that would damage his political career. He was forced to find another seat at Bethnal Green South West, for which he was elected at a 1911 by-election. But when he was appointed to the cabinet in 1914, by the convention of the time he had to seek re-election at a by-election, which he duly lost. He then began an unsuccessful quest for an alternative seat, losing another by-election at Ipswich, before ultimately being forced to stand down from the cabinet.
Perhaps if he had employed a more competent agent at West Ham, he would have avoided all the later trouble.
Jonathan Calder at Liberal England has written extensively about Masterman here.