In 2006, the Democrats boycotted the snap election in Thailand. One of the problems was that because of the boycott not all seats were filled. Xinhua in April 2006:

The Election Commission (EC) of Thailand announced on Monday that Sunday’s by-elections failed to fill at least 14 parliamentary seats, mostly where single candidates failed to get the required 20 percent of votes. The EC refused to give details on the turnout rate and the size of the protest vote.

As of Monday morning, the EC said, the governing Thai Rak Thai party won 15 seats, while tiny fringe parties took eight. Three districts had yet to report their results. The other 14 elections were non-conclusive.

The result opens a potential constitutional crisis since the constitution requires the parliament session must include all 500 MPs without any absence.

Election Commission spokesman Ekachai Warunprapa said the EC would meet Monday to decide whether new by-elections for the 14 empty seats could be held this week.

BP: The problem was that The Organic Act on The Election of Members of the House of Representatives and Senators 1998 and Section 74:

In any constituency, if, on the election day, there is one candidate standing for an election on a constituency basis and such candidate receives votes at least twenty percent of the total number of voters in that constituency, the Election Commission shall announce such candidate to be the person elected.

In the case where the only candidate standing for an election under paragraph one receives votes of less than twenty percent of the total number of voters in that constituency, the Election Commission shall hold a new election in such constituency.

Source: Word  DOC

BP: Essentially, the winner who runs unopposed needs 20% of the vote. Because of the boycott, Thai Rak Thai was not able to win all seats. There were at least one round of by-elections were a Thai Rak Thai candidate ran again, but still they were unable to get 20%. As not enough seats were filled, parliament was unable to convene on time as there was no quorum. This is what led HM the King to state it was a mess and the Court to invalidate the election.

Can the same thing happen this time?

Essentially, No.

Section 93 of the Constitution after the 2011 amendment:*

The House of Representatives consists of five hundred members.

In the case where there occurs, during the general election, any cause resulting
in the members of the House of Representatives being less than five hundred in
number but not less than ninety-five per cent of the total number of members of the
House of Representatives, such members are deemed to constitute the House of
Representatives. In this case, the acquisition for the fulfillment of the total number of
members of the House of Representatives shall be completed within one hundred and
eighty days and the new coming members shall hold office for the remaining term of
the House of Representatives.

BP: So we need only 95% of the 500* MPs for there to be a quorum

Section 88 of the new Organic Act:

Section 88 In any constituency, if, on the election day, the number of candidate or
candidates standing for an election on a constituency basis is equivalent to or less than the
total number of Members of the House of Representatives of that constituency, a candidate in such constituency is elected only when he or she receives votes of not less than twenty percent of the total number of voters in that constituency and more than the total number of ballot papers casting a non-voting. In the case where such candidate has received votes of less than twenty percent of the total number of voters in that constituency or not more than the total number of ballot papers casting a non-voting, the Election Commission shall order a new election only for the positions having received votes of less than twenty percent of the total number of voters or not more than the total number of ballot papers casting a nonvoting.

The provisions of Section 9 shall apply mutatis mutandis.

In a new election under paragraph one, if there is a number of candidate or candidates
equivalent to or less than the total numbers of vacancy or vacancies to be filled, the
provisions of paragraph one shall apply mutatis mutandis. And if any candidate has again
received votes of less than twenty percent of the total number of voters or not more than the total number of ballot papers casting a non-voting, the Election Commission shall order a new election once again according to the provisions of paragraph one.

In a new election under paragraph two, if there is a number of candidate or candidates
equivalent to the total number of vacancy or vacancies to be filled, the candidate or
candidates who has received the largest numbers of votes shall respectively be elected only to fill the vacancy or vacancies. The Election Commission shall announce the results of election thereof within the period of time as prescribed in Section 8.

Source: EC

BP: From BP’s reading, for the first two tries, the winning candidate needs 20% of the vote, but if we go to the third by-election then the winner wins even if they don’t break the 20% threshold.

Also, Puea Thai is not the only party running for the February 2, 2014 election. So far it is only the Democrats who may boycott this election, but there is no sign that the smaller parties will boycott. In 2006, all the major smaller parties boycotted. Hence, the boycott threat of the Democrats only really works if they can somehow delay the election for another reason because if the election on February 2 goes ahead and the Democrats don’t participate, there will be another reason for invalidation as the reasons from 2006 won’t work. The Democrats need to take this into consideration regarding the boycott…

NOTE: There are some other factors for the Democrats to consider so this post is quite narrow in focus.

*UPDATE: Revised post to change from 480 to 500 and new link with revised wording of the constitution. Only 95% quorum part still applies.