I should correct myself, I was too general.
Excessive, EXCESSIVE cardio combined with weights can lead to a state of overtraining. Also, cardio can contribute locally to fatigue in heavily involved muscle groups.
Well-designed cardio should be aerobically taxing but not muscularly taxing. For instance, swimming.
Some cardio IS muscularly taxing, like cycling. That probably will fatigue your legs, although it won't contribute to overtraining at normal levels.