Scotland's exams body has admitted that a missing letter in a web address caused problems for candidates trying to access results.
About 35,000 pupils registered to receive Higher and Standard Grade results online a day early.
However, some received error messages when they tried to enter the site using web links supplied by the Scottish Qualifications Authority (SQA).
The error was caused by a letter "s" missing from an SQA web link.
Dozens of candidates contacted the BBC Scotland news website to say they were unable to access their results online on Monday, a day before results were due to arrive in the post.
A number of disgruntled candidates and their parents said problems occurred because they were initially directed to the non-secure web address which has the prefix http, instead of the secure site at https.
They said the incorrect link was supplied in letters to candidates who had requested to receive their results online.
The SQA confirmed there were problems with the link. It added that more than 3,000 people had also forgotten their login details.
Spokesman Mike Haggerty said investigations into the possible causes of log-in difficulties were ongoing, but admitted it was clear the SQA log-in address error had contributed.
Mr Haggerty said the problem arose when the extra "s" was added to the link to make the site more secure - but only after letters giving the non-secure link had already been sent to candidates.
He said it was impossible to know how many students had been affected by the error as not all pupils who registered to receive their results online would have attempted to log-in on Monday morning.
Mr Haggerty added: "We are sorry for the difficulties experienced by candidates. We will learn the lessons from this year and will continue to work to improve in the future.
"We would like to thank everyone involved in this new system for their patience and support."
Anna Kordeczka, 17, a former Mearns Castle High School pupil, spent three hours trying to get her Higher results.
She said: "The web page on the letter issued to all candidates is wrong.
"This is no doubt the reason why so many problems have been encountered."
Liz Smith, the Scottish Conservative spokeswoman on children, schools and skills, said the SQA must make sure the system works properly next year.
Judith Gillespie, of the Scottish Parent Teacher Council, who is also an SQA board member, said lessons would be learned.
She said: "There were a series of problems all of which have to be looked at.
"But this was a bold an innovative step which will bring huge benefits in the future."
About 6,000 results were sent successfully by text message.
A further 120,000 students preferred to wait until Tuesday, when their results were being delivered by post.
According to the SQA, the percentage of students passing Higher exams has increased since last year.
The Royal Mail said it had delivered a record number of exam results on behalf of the SQA.