The plaintiffs argue a case of false advertising that could be potentially dangerous to a user's health -- citing multiple claims of instances where a user's heart rate was significantly higher than what the device reported. Specifically, the court documents describe a case where a user's personal trainer recorded their heart rate at 160 bpm, but the device only read 82 bpm. If the user had continued with hard exercise due to the under-reported rate, they might have received the maximum recommended heart rate for their age, putting their health in danger.
Speaking with Ars Technica, Fitbit largely dismissed the claims, stating that it stands behind its technology and, moreover, that Fitbit's products are not medical devices. A fair point, maybe, but as a fitness product these trackers do walk a fine line. Either way, maybe you should think twice before taking your wearable's data at face value.