We often deal with this question. Devotees miss out and end up taking grains on Ekadashi. There is always a debate whether we need to fast on Dwadashi or Trayodashi ? ( heated debates among devotees :) grain leading to Migrain )
An instance of this kind comes up in TKGs (Tamal Krishna Goswami's )diary
May 13
Today was Ekadasi, but we committed the grievous error
of eating grains. We found out too late. Srila
Prabhupada chastised Pradyumna, "What is the use of
having you come with us? Is this not your duty?" We
were ordered to observe Ekadasi tomorrow and to go on
preparing grains today. Prabhupada commented, "Big,
big monkeys; big, big belly."
So we can fast on Dwadashi .
I also got the following material from http://www.stephen-knapp.com/ekadasi.htm . This website is iskcon oriented and is maintained by one of Srila Prabhupadas disciple.
"It is said that even if one mistakenly misses the observance of an Ekadasi, he or she may make up for it by observing it the very next day on Dvadasi, and then break one’s fast from grain on the next day, Trayodasi. One may also observe the special fast on Nirjala Ekadasi. This is also called Bhima Ekadasi. This is because the Pandava brother known as Bhima was so strong and had such a voracious appetite that he could not observe Ekadasis twice a month. He could not fast because he was too hungry. So Lord Krishna told him to merely observe one Ekadasi a year, which is the Nirjala Ekadasi. Nir jala means no water. So he had to observe at least one Ekadasi a year, and on that day he had to abstain from not only beans and grains, but from all foods, even water. So devotees who miss an Ekadasi day often observe a complete fast from all food and liquids on the Nirjala Ekadasi, which is usually sometime in June, and thus make up for whatever was missed. However, this is a very potent Ekadasi, so a complete fast on this day gives one who observes this many pious credits. "
All glories to Srila Prabhupada who has helped us out in all the occasions
An instance of this kind comes up in TKGs (Tamal Krishna Goswami's )diary
May 13
Today was Ekadasi, but we committed the grievous error
of eating grains. We found out too late. Srila
Prabhupada chastised Pradyumna, "What is the use of
having you come with us? Is this not your duty?" We
were ordered to observe Ekadasi tomorrow and to go on
preparing grains today. Prabhupada commented, "Big,
big monkeys; big, big belly."
So we can fast on Dwadashi .
I also got the following material from http://www.stephen-knapp.com/ekadasi.htm . This website is iskcon oriented and is maintained by one of Srila Prabhupadas disciple.
"It is said that even if one mistakenly misses the observance of an Ekadasi, he or she may make up for it by observing it the very next day on Dvadasi, and then break one’s fast from grain on the next day, Trayodasi. One may also observe the special fast on Nirjala Ekadasi. This is also called Bhima Ekadasi. This is because the Pandava brother known as Bhima was so strong and had such a voracious appetite that he could not observe Ekadasis twice a month. He could not fast because he was too hungry. So Lord Krishna told him to merely observe one Ekadasi a year, which is the Nirjala Ekadasi. Nir jala means no water. So he had to observe at least one Ekadasi a year, and on that day he had to abstain from not only beans and grains, but from all foods, even water. So devotees who miss an Ekadasi day often observe a complete fast from all food and liquids on the Nirjala Ekadasi, which is usually sometime in June, and thus make up for whatever was missed. However, this is a very potent Ekadasi, so a complete fast on this day gives one who observes this many pious credits. "
All glories to Srila Prabhupada who has helped us out in all the occasions