The issue: When you're tired or unfocused, you compromise your form, and that's when injuries happen, says Jessica Matthews, an exercise physiologist for the American Council on Exercise. Just one sloppy rep, especially when holding heavy weights, can cause you pain or discomfort for weeks. Over time, pushing yourself too hard—or letting someone else push you—can lead to serious overuse injuries (like in this woman's case).
The fix: Tell your trainer as soon as you feel dizziness or pain (even soreness can be a problem if it's affecting your form) or if you're seriously dragging. You trainer won't hold it against you—in fact, it's in his best interest to keep you healthy, safe and active.
Next: The 6 most dangerous workout moves