Hundreds of would-be voters in several UK locations have complained that they were denied the chance to vote before polling stations closed at 2200 BST.
How widespread have the problems been?
All of the problems reported so far have been in England. Police were called to several incidents in London, including in Hackney and Lewisham, and there were other incidents in Surrey, Manchester, Birmingham, Liverpool, Leeds, Newcastle and Sheffield.
What caused them?
The main reason appears to be a higher-than-expected turnout, creating long queues late in the day and, some say, insufficient council staff to cope.
Presiding officers in Sheffield also blamed people turning up to vote without their polling cards. While these are not required, they said it slowed down the voting process as it took longer to check people's details.
One also blamed the "absolute laziness" of the electorate, saying many people were not prepared to join queues earlier in the day.
In Sheffield Hallam, Liberal Democrat leader Nick Clegg went to apologise to hundreds who missed their chance after queuing for hours. The returning officer there said it was probably the highest turnout in 30 years, and with a lot of new voters, council staff were "caught out" by the scale of the operation.
A lack of ballot papers at one Liverpool polling station meant some voters were left waiting until more arrived.
What does the law say?
The law says polling stations are open from 0700 to 2200 and no ballot papers are issued after that time, although everyone who has a ballot paper by then must be able to vote. If you do not have a ballot paper, but are inside the polling station, you cannot vote.
At one polling station in Lewisham ballot papers were handed out to people in the queue before the 2200 deadline.
While in Newcastle, about 450 people queuing outside two polling stations were taken inside and allowed to vote after the deadline passed, a city council spokesman has confirmed.
What does the Electoral Commission say?
The commission said it was a "serious concern" that many people were unable to vote, and that the law is "extremely clear" on who can vote up until 2200.
It also said that there should have been sufficient resources to ensure that everyone who wished to vote could do so.
The Commission said responsibility to make sure there were sufficient polling stations - and to decide the number of electors allocated to each station - lies with the returning officer in each constituency.
What will happen next?
The Electoral Commission has launched a review of what happened. It has asked members of the public who were unable to cast their vote because of queues at polling stations to let them know about their experiences
using this online form.
Its review will look at the planning and management of local returning officers, their specific response to problems encountered, and "the impact of guidance, advice or support" provided by the Commission.
It aims to publish an interim report by the end of the next week, with the full report due to follow in July.
The Commission itself does not have the power to overturn election results, or order a re-run. The onus is instead on voters to launch a legal challenge.
How does someone launch a legal challenge?
Firstly, you need to issue an election petition to the Election Petitions Office. This has to be done within 21 days of the general election.
Once you have issued your petition, you then have three days to pay the £5,000 fee, or at least show you have the means to pay. The decision on whether you have to pay upfront is made by a senior official at the Election Petitions Office.
After your petition is issued, you then have 28 days to go ahead and make an application to the High Court.
Two High Court judges would then rule on the application, and whether another election should be held in the constituency in question.
This is no timetable for when this court hearing has to take place, but a spokeswoman for the Election Petitions Office says "the over-riding objective is to get it dealt with quickly".
If the application is successful, the person who made it gets his or her money back. If unsuccessful, then they would lose their £5,000.
Human rights lawyer Geoffrey Robertson QC has also said that people denied the right to vote could sue for compensation.
He estimates that individuals could get "at least £750" because under the European Convention you have a right to vote.
Should the way in which we cast our vote be modernised?
Some commentators have said that the problems experienced in a number of constituencies show that the process by which the UK votes in general elections is now antiquated and needs updating.
"What we currently have is a 19th Century process for a 21st Century population," said Ovum analyst Mike Davis.
"A 24-hour society requires 24-hour voting to meet the aspirations of the electorate, who now have an expectation of 'always on and always available'."
While some are calling simply for polling stations to be kept open much longer, or for more of them, others suggest that in this internet age people should be able to vote online.
However, fraud and security concerns are likely to continue to rule out remote, electronic voting.