Sometimes good intentions cause bad things to happen. In this perfect paceline (evenly spaced, slightly staggered left-right-left), the last rider hears a car behind and believes it’s necessary to warn the others. So far, so good. But the well-intentioned warning could create disorder at the worst possible moment if not all riders know how to ride a paceline correctly. 
Hearing “car back!,” one of the riders reduces pedal pressure—a completely unneccesary and thoughtless move (often made worse by the shout “slowing!”) The rider behind taps the brakes, and the accordion effect forces the next rider to brake hard. The last rider now has the interesting choice of either locking up the brakes or swerving around the chaos in front. 
 The last rider decides to swerve around the bunch. Unfortunately, the car he warned the others about is approaching fast. Now the well-being of all riders depends completely on the skill of the driver. It’s a worst-case scenario, but it could happen. Lesson: don’t reduce pedal pressure when you hear “car back.” Or “car up,” for that matter. Just keep riding!