halacha - If you check for bugs according to halachic requirements, and then there is a bug in the food, are you liable?

If one or one's wife does everything that is needed for checking, but one accidentally eats a bug anyway, is one liable for the sin(s)?

I'd also be interested in knowing whether one would be liable if one ate a bug that was in a hechshered product, such as pre-checked lettuce or kosher frozen vegetables.