Any time the cart is invoked in regards to a training camp injury, it causes people to fear the worst. But in certain cases, with certain players, the panic might be justified.
According to Adam Teicher of the Kansas City Star, Chiefs running back Jamaal Charles just left the field on a cart after huddling with trainers.
Kansas City Chiefs insider Reid Ferrin says that Jamaal hurt his foot and that X-Rays are negative.
Coach Reid says Jamaal hurt his foot X-rays negative
— Reid Ferrin (@KCChiefs_Reid) August 12, 2013 
Source: Darin Gantt, NBC Sports Pro Football Talk