Once in a while, a timing chip is not successfully scanned, sometimes at the start line, sometimes at the finish line, sometimes both. Some of the reasons a timing chip scanning failure might occur include:

• It is worn on the back when it is the front that is seen by the scanning antennas. (Upper chest is preferable for the bib number.)
• It is covered. (Wearing the bib number on your tummy increases the risk the timing chip will be covered when you tie your jacket around your waist.)
• It is too close to the body. (The water in a human body (and the related perspiration) can absorb the radio waves. Weaker signal, weaker chip scan. Preference is for no more than three safety pins being used for the bib number (two is better -- one on a top corner, one on the diagonal opposite corner), thus allowing the timing chip to be just a bit further away from the body.)
• It has been scrunched. (Some track and field athletes crumple their competitor numbers as soon as they receive them because a "pressed" competitor number is too uncomfortable/geeky for them.)
• It is wet due to external conditions. (Different timing chips are used for triathlons and "mud runs".)
• It is being blocked by GPS watches. (The classical "Start your watch at start line..." and "Stop your watch at finish line..." moves (in which one arm is across the chest) can cause the timing chip to not be scanned successfully for that start/finish.)
• It is being blocked by another body. (Starts having more space between each competitor and the athlete ahead of them allow for a higher rate of timing chip scanning success.)
• It is too close to a transmitter, such as a heart rate monitor strap. All transmitters generate spurious emissions (i.e.: the transmitter sends noise out on frequencies other than the one it is supposed to transmit on). Any transmitter that is close to a timing chip could cause an issue.