Greece's prime minister appears to have thrown a spanner in the works of the project to rescue the Greek economy - and arguably the whole eurozone - by announcing he will put a hard-negotiated bailout package to the Greek people in a referendum.
It is a risky step, given the strength of popular opposition to the deal, and George Papandreou's opponents say a "No" vote would probably force Greece out of the euro and could trigger wider contagion within the eurozone.
The history of the European project is dotted with the carcasses of referendums that threatened to halt it in its tracks.
Here are some occasions on which European populations have given "the wrong answer".
The Danish public were a thorn in the side of the treaty which created the European Union and the euro.
The 1992 Maastricht Treaty was to create the European Union and the single European currency, but it required the ratification of all member states. Permission to ratify was withheld by Danes in a 1992 referendum.
A scramble to save the project ensued, resulting in the Edinburgh Agreement, which granted Denmark four exceptions to the treaty, including on adopting the euro.
In 1993, a fresh referendum saw the treaty ratified and put back on track.
In 2000 the government held a referendum on introducing the euro, which was defeated.
The 1999 Treaty of Nice, which amended the Maastricht Treaty, sought to prepare the institutions of the EU for its anticipated eastward enlargement.
Ireland was the only one of the then 15 member states legally obliged to put the treaty to its citizens in a referendum and, following a low-profile campaign, in a shock result they rejected it.
Only 35% of voters turned out, prompting some Irish politicians to call for the end of the practice of putting each new treaty to the vote.
It looked like a serious blow to the European project, as foreign ministers rejected any initiative to renegotiate the treaty text.
But just over a year later, another referendum was held - this time accompanied by a massive "Yes" campaign - and the treaty was passed.
Netherlands and France 2005
In 2005, the EU launched a mammoth consultation to secure approval of a new, 485-page constitution - which was supposed to "streamline" European decision-making by consolidating EU treaties in a single text.
Again, this required the approval of what were then 25 member states - and 18 ratified the text, with referendums held in two of those countries, Spain and Luxembourg.
But referendums in France and the Netherlands in May and June 2005 were lost, striking a deadly blow to this constitutional project which had been three years in the making.
A fatal blow may have been struck, but in 2007 the project to rationalise the running of the EU stirred into life again, this time in the guise of the Lisbon Treaty.
This tome was as weighty as the original constitution, and preserved the substance of it, though its authors argued it differed in key ways.
The final draft of the text was agreed in October 2007, and most countries argued referendums were unnecessary, as this time it only amended, and did not replace, previous treaties.
But Ireland was again legally obliged to hold a referendum, and in June 2008 voters once again gave constitutional architects the knock-back, rejecting the treaty by a margin of almost seven percentage points.
Immediately, EU foreign ministers vowed to keep the Lisbon project alive, and 16 months later the treaty - now containing what Irish leaders described as important safeguards - was once again put to Irish voters.
This time it overwhelmingly passed.
There was an air of deja-vu about the whole episode, which mirrored Irish voters' treatment of the Nice Treaty referendum in 2001.
Iceland 2010 and 2011
Iceland may be on the fringes of Europe, and outside the EU, but its dramatic banking collapse in 2008 shook the heart of the continent.
Icelandic voters have, in referendums in 2010 and 2011, rejected settlements with private banks to reimburse investors in the Netherlands and UK, who lost 4bn euros (£3.5bn; $5.8bn) with the collapse of Iceland's Landsbanki bank.
Iceland's prime minister rued the votes, which extended Iceland's enforced stay in the financial wilderness and left a key issue of the 2008 turmoil outstanding.
Observers said they were unlikely to assist in Iceland's bid to join the EU - on which the Icelandic people seem set to face another referendum.