Ошибочка: mistake, minor or major.
I've detected a slight buzz of panic among Moscow's expat community. It seems that a Russian work visa or residence permit will only be issued to those of us who can pass a test on Russian language, culture, history, and even legislation. Кошмар! (What a nightmare!)
To help you cram (зубрить), here is a sample multiple-choice (тест) question: Что такое косяк? (What is косяк?) a) a door jamb; b) a group of birds or fish; c) something crooked; d) a mistake; e) none of the above; f) all of the above. If you guessed "f" — Садись! Пятёрка! (Sit down. You get an A). You will pass your exam with flying colors.
Косяк is a funny word. In it you hear косо (crookedly), and yet it refers to something that must be straight: Он беспомощно привалился спиной к дверному косяку (He helplessly fell against the door jamb). Or something that would seem to have nothing to do with straight or crooked: Посетители шли в театр косяком (People swarmed into the theater). Or something that is either up in the air or down in the sea: Летал над нами крикливый косяк журавлей (A noisy flock of cranes flew over us). Вышел косяк лещей и плавно двинулся в сторону берега (A school of bream appeared and smoothly swam toward the bank). And the slang usage is equally rich. Косяк might mean a joint (as in marijuana) or a face (as in ugly mug), but since none of my friends are gangsters, I can't confirm it. I can, however, testify that косяк means a goof or gaffe, because I hear, read and use it that way all the time. Ой! Забыла предупредить, что приду сегодня позже. Мой косяк (Oops! I forgot to warn you that I'd be late today. My bad).
Russian has other words for goofs. Ошибка (mistake) is the most neutral. Its diminutive, ошибочка, might mean a minor error: Никого не шокирует стилистическая ошибочка (No one is shocked by a little stylistic error). Or it might mean a great, walloping, horrible mistake that you are trying to downplay. This is a classic linguistic ploy of teenagers: Папа, ошибочка вышла. Я думал, что нажимал на тормоза, а оказывается, что нажал на газ. В дерево врезался. (Dad, I made a little boo-boo. I thought I was hitting the brakes, but it turns out I was pressing the gas pedal. I rammed a tree). Another word easily recognizable to English speakers is ляпсус (lapse). It is even more comprehensible to anyone who studied Latin in school, since it is a transliteration of lapsis and is used in much the same way — most often to describe a typo or slip in speaking or printing.
Почерк автора был неразборчивый, и некоторые ляпсусы попали в первые публикации (The handwriting of the author was illegible, so several mistakes made their way into the first publication). Ляпсус has a short-form goof, ляп, which is slangier and a bit more universal. В принципе, фильм можно было бы считать удачным, если бы не несколько ляпов (Overall, you could say the film was successful except for a few blunders).
If you prefer guns to dead languages, you might like the word промах (miss, slip-up) to describe your goofs. Забыл купить подарки и кинулся исправлять свой промах (I forgot to buy gifts, so I rushed to fix my blunder). And if you come empty-handed, just say: Мой косяк.