It is a popular expression used to indicate an oversight or mistake. Negligence, error, carelessness, mistake.
familiarly used to designate an error arising from carelessness or lack of meditation.
Sustantivación of the expressive beginning " thought to " which is usually disculpatorio of a mistake, negligence or carelessness in the performance of an action, that is the saying goes in Castile: worth more a posiacaso than a hundred penseques.