Here are several examples of "fitness" gone too far:
For Drug Guys
If you're sporting a nice shade of magenta all the time and rocking a tomato face, your dose is detrimental.
Also, count the number of bodybuilding drugs you're taking. Now count the number of other drugs you're taking to stop the side effects of those bodybuilding drugs. If the second number is greater than the first, that's detrimental.
For Drug-Using Females
If you have facial hair and there are parts of your body that are enlarged that you don't flex on stage... that's detrimental.
If you refuse to date someone because they don't total a certain amount, emotionally it's become detrimental.
If you're into anything fitness related that causes you to abstain from sex or withhold it from your partner for any and all reasons, it's become beyond detrimental. This includes avoiding certain sexual positions because you believe that it will widen your waist, taking away from your V-taper. If you won't perform in the missionary position because "it's bench day tomorrow" it's become detrimental.
Likewise, if you create body image disorders in your kids because you obsess too much over what you look like, it's become detrimental. And if the only time you can have any semblance of control over your eating habits is when you're preparing for a show, seek help.
If you find you've sacrificed friendships, romantic partners, or jobs because they interfered with training or competing, it's become detrimental.
Finally, if you end up believing that your worth as a man or woman somehow lies in the makings of a plastic trophy, or believe that your identity revolves around a powerlifting total or arm measurement... you suck at life. No really, you suck at life.
Fitness, training, competing, dieting, whatever. All of it can be done while still finding balance in the rest of your life. Most of the time these things become detrimental when people can't find an identity outside of it. They have lost all logical ability to keep these things in perspective. Virtually every aspect of their lives revolve around it.
When I give seminars I usually end them by talking about balance. I relate a story once told to me by a very experienced lifter who was about to walk out of the house to go squat. His young daughter came up to him and said, "Daddy, would you have a tea party with me?" He said, "I dropped my bag and missed that squat session because in twenty years I'd never remember that workout. But I'd always remember the tea party with my little girl."