Is there any such thing as a bad meditation?

I've always been told that there is no such thing as a bad meditation. No matter what happens it is OK. In fact the meditator is a bad judge of the effectiveness and efficacy of any sit even if those concepts did have meaning. However I was reading Everyday Zen by Charlotte Joko Beck and she said that she encounters people who have been meditating who are frankly wasting their time. She says they may as well have been practicing their golf swing for 20 years. Ooohhhh harsh. So what's going on here? Is there really no such thing as a bad meditation and every sit is just grist for the mill. Or is it possible to waste your time and potentially waste years. Is 'there is no such thing as a bad meditation' just something you tell beginners to motivate them - a kind of expedient means?