There are several actions that will result in skaters being disqualified from a race, and having their time rendered invalid.
Impeding: Pushing, blocking, tripping or otherwise causing an impediment for another skater
Off track: Skating outside the designated track
Assistance: Giving physical assistance to another skater. For example: pushing a teammate from behind for an extra boost, or allowing a teammate to lean on another for stability in corners
Shooting the line or Kicking out: Driving the foot in lead ahead to reach the finish faster, resulting in the lead foot lifting off the ice and creating a dangerous situation for others
Unsportsmanlike conduct: Acting in a manner not befitting an athlete or a role model. Including cursing at a competitor, kicking your feet, striking other skaters or officials, etc.
Equipment: Not wearing the proper safety equipment, losing equipment during the race, or exposure of skin not on face or neck.
False Start: Leaving before firing of the starter's pistol. On the second violation in the race, the offender on that start is disqualified.
Did not finish: Usually due to injury, the skater did not finish the race.
Did not skate: The skater did not go to the starting line.