The Liberal Democrats have secured a major upset in winning the "safe" Labour seat of Dunfermline and West Fife.
But the party is by no means a novice when it comes to by-election success.
Willie Rennie's is the latest in a long line of Liberal upsets
During the last 25 years, the Lib Dems - or their predecessors the Liberal Party, the SDP and the Alliance - have taken 19 seats from other parties in this way.
The BBC News website takes a look at some of the biggest upsets:
1981 - CROYDON NORTH WEST
The parliamentary career of Bill Pitt famously began with a complaint on live television - immediately after his by-election victory - that someone had sprayed a bottle of champagne all over him.
"That's my new suit!" he complained.
When the Conservative MP for Croydon North West died, the local Liberal Party selected Mr Pitt - who had stood in the three previous general elections - as its candidate for the by-election.
The Liberals had recently formed an alliance with the Social Democratic Party (SDP) which wanted to put its own candidate up for election instead of Mr Pitt.
The SDP saw the by-election as the perfect opportunity for former Labour cabinet minister Shirley Williams, who had lost her seat two years earlier, to return to Parliament.
The Liberals stood their ground with Mr Pitt - standing as a "Liberal with SDP support" candidate - ousting the Tories on a 24.2% swing with a 3,254 majority.
1983 - BERMONDSEY
The swing achieved by Simon Hughes, then 31, in 1983 was beyond his and his supporters' wildest expectations.
After a taking on gay Labour candidate Peter Tatchell, Mr Hughes ended almost 50 years of Labour rule with the biggest swing since the war.
He did this by pushing up the Liberal vote by more than 50%.
In doing so, he took 57.7% of the total vote leaving Mr Tatchell with 26.1%.
When Mr Tatchell's predecessor Bob Mellish took the seat in May 1979 he had achieved 63.6% of the vote.
In the bitter battle for victory, the Liberals, in one leaflet, had presented Mr Hughes as the "straight choice".
Mr Hughes, who later admitted to having had gay relationships, apologised for parts of his campaign which may have been homophobic.
1987 - GREENWICH
Greenwich was the by-election which revived the SDP/Liberal Alliance and undermined Labour just four months before a general election.
When the Labour MP for the constituency Guy Barnett died at the end of 1986, the local Labour Party put forward a left-wing candidate.
Rosie Barnes, with the support of a mass of Liberal canvassers, was thrust into the political spotlight to take Labour on.
The upset, which came four months ahead of the next general election, was disastrous for Labour.
Ms Barnes won the seat with a 16.1% swing in the Labour stronghold.
2003 - BRENT EAST
Sarah Teather's impressive victory in this usually safe Labour was the party's first Commons by-election loss since its return to power in 1997.
Moreover, it was also Labour's first by-election defeat in 15 years.
Ms Teather, a press officer and former councillor for Islington, received 8,158 votes winning the poll by more than 1,100 votes.
At the age of 29 - and having achieved a massive 29% swing - she became the youngest MP in the Commons.
Remarkably, at the 2001 general election, the Liberal Democrats had come third in the constituency, taking only 7.8% of the vote.
Opposition to the Iraq war was one of the key issues on which Cambridge graduate Ms Teather fought the by-election, amid concerns among the constituency's large Muslim community.
Her defeated Labour opponent conceded that the "backdrop of the controversy over Iraq" had not helped.