Only occasionally, and it seems to be when the HR sensor had difficulty getting a reading during a run. I have noticed that the HR sensor has difficulty in colder weather. For me, it starts to become less reliable below about 45 degrees Fahrenheit. If I tighten it up by on hole on the sport strap, it gets better. I have not taken it below 35 degrees yet, so I do not know if there is a lower threshold temperature at which the sensor becomes unreliable regardless of how tight the band is.