According to Ms. Ryan in First Confession, what did it mean to make a bad confession?

In the early stages of the story, Mrs. Ryan tells the
narrator a tale of a man who made a "bad confession." In the story, the man arrives at a
priest's bedside late at night, asking for the chance to confess; the priest
uncomfortably asks him to return in the morning, but the man explains that "last time he
went to confession, there was one sin he kept back, being ashamed to mention it, and now
it was always on his mind." The priest is said to have gotten up and dressed
immediately, now aware that the man had made a "bad confession." This is then defined as
a mortal sin. In other words, Mrs. Ryan is suggesting that a bad confession is one which
is incomplete, and therefore a lie by omission in a sacred process. Had the gentleman
fully confessed, it would not have been tarnished; however, in lying while allegedly
confessing, he essentially spits in the face of the priest, the doctrine, and God
himself, according to most Catholics. The woman obviously considers this omission a
greater sin than any he might have needed to confess in the first place, simply because
he refused to spout it and it preyed upon his mind.