What should I do if I accidentally break Ekadasi?
Srila Prabhupada recommended that if we inadvertently eat some grain on an Ekadasi, we should observe the Ekadasi fast for the rest of that day and also on the next day, Dvadasi. If even that is not possible, we should simply go on with our activities in Krishna consciousness and pray for forgiveness.
Some fast completely on Pandava Nirjala Ekadasi to atone for breaking Ekadasi during the previous year. Certainly there are benefits for observing a total fast, but none of our information supports fasting on Pandava Nirjala Ekadasi to make up for breaking other Ekadasis. For more detailed information on this, you may wish to visit the Diary of Bhakti Chaitanya Swami.