There have been very many of those over the years.
At other times they are an early harbinger of change that is to come.
A warning to a party in Government that their electorate is angry that they are not fulfilling the commitments they made, or the beginning of change and the emergence of a new direction in politics.
Those are few and far between.
But there is no doubt it has set the political pundits’ minds whirring.
This is a seat in the heart of Conservative Buckinghamshire that the party has held for a century. In every previous general election the Conservatives had won a majority of the votes with winning margins between 10 and 23 thousand.
There was no suggestion that the 16,000 majority achieved by the late Dame Cheryl Gillan would be under threat when the date was announced.
And yet here we are with not just a dramatic Liberal Democrat gain by Sarah Green, but one which was delivered with a stonking 8 thousand majority and a swing to the party which almost defied belief.
Cynics will of course, and with no hint of irony, point to the Lib Dems’ history of success in by-elections over the years.
There was Eastleigh in 2013 when the party defied common expectation to hold on to the seat of disgraced former cabinet minister Chris Huhne.
Willie Rennie’s spectacular win in Dunfermline and West Fife in 2006.
Or most recently Jane Dodds’ capture of Brecon and Radnorshire from the Tories in Summer of 2019.
Of course what each of them have in common was that the success only lasted until the next general election.
When it comes to Chesham and Amersham I see much greater parallels with both another Lib Dem win and one by a different party in a different era.
In 2003 a young Sarah Teather snatched the formerly safe seat of Brent East from Labour.
At the time the Guardian newspaper described it as ‘sending a shock wave’ through Tony Blair’s government and delivering a boost to Charles Kennedy and the already growing support for the Liberal Democrats’ stance against the Iraq war.
Sarah’s s success was repeated in two subsequent General Elections before she stepped down in 2015.
But the significance of that 2003 by-election win was that it came when a governing party, having secured re-election, was facing criticism from its own supporters, discontent on its own benches and national disappointment that would eventually knock it from office.
In Chesham and Amersham over the past few weeks those of us knocking doors very quickly discovered that this was a seat out of love with its former Tory partner.
The chaos of the HS2 construction bulldozing its way through the previously scenic serenity of the Chiltern Hills was a major cause of discontent.
But even more widespread was a genuine mood of abandonment.
A feeling that these were communities whose attachment had been taken for granted, and needs ignored for too long.
There was a national problem too.
The litany of incompetence, lack of transparency, chaotic mismanagement and general incohesion of Boris Johnson’s administration is driving people away from his party even in a part of the country where they are omnipresent, and he was once an MP.
It was all very reminiscent of the fading allure of the Labour government.
The blue wall is now as much in danger of disintegration as the red one.
Which brings me to that other example of a by-election victory that was a hint of what was to come.
Anyone who knows me will know that high on the list of politicians I admire is the late incomparable Margo Macdonald.
That may partly be because I remember the shockwaves her 1973 victory in the Labour stronghold of Govan sent through the Scottish political establishment.
It was certainly a short-lived success but the reason that it differs from other temporary constituency occupancies is that this was the first indication that the red stranglehold of Glasgow, indeed Scotland, could be vulnerable.
It was not the SNP’s first by-election victory, nor their last significant one, and it was several years before we saw the full picture emerge.
But it is the one that many of us look back on as the moment when the Labour party should have wakened up and started smelling the coffee.
At that time they regarded Glasgow as their impenetrable heartland in much the same way as the Tories do the Home Counties now.
But as the latter has increasingly focussed on challenging Labour in the North of England it has become potentially vulnerable to disgruntled voters in the South.
Those are the very people who turned to the Liberal Democrats and Sarah Green this past week.
There are another 80 or so seats across that region where the Liberal Democrats came second to the Tories in 2019.
Some much closer than the 16 thousand vote cushion Boris Johnson thought his party had in Chesham and Amersham.
Lib Dem leader Ed Davey claimed on Friday that the by-election victory is the first sign of the crumbling of the ‘Blue Wall’.
Time will tell if he’s right.
Chritine Jardine is the Scottish Liberal Democrat MP for Edinburgh West