Mortal sins are a bit more vague. They are voluntary acts "said, done or desired contrary to the eternal law, or a thought, word, or deed contrary to the eternal law." The key word there is "voluntary" because as this site puts it, mortal sins cannot be committed "accidentally." That's good news, but according to Wikipedia, mortal sins cannot be forgiven after the sinner dies. Depending on your particular system of beliefs, that may be cause to sign up for some volunteer work sooner rather than later.