March 23: A controller on his fourth consecutive overnight shift at Washington, D.C.'s Reagan National Airport left the radio tower silent after apparently falling asleep. Two commercial airliners were forced to land on their own.
March 29: Two controllers at Preston Smith International Airport in Lubbock, Texas, did not hand off control of a departing aircraft to another control center and it took repeated attempts for them to be reached.
April 11: A controller at Boeing Field/King County International Airport in Seattle fell asleep on the job. Boeing Field does not handle any commercial air travel.
April 13: A controller at the Reno-Tahoe International Airport in Nevada was sleeping as a plane carrying a crticially ill patient was trying to land.
April 16: An air traffic controller fell asleep on the job at an air route control center in Florida.
April 17: An air traffic controller near Cleveland was suspened after being caught watching a movie -- Samuel L. Jackson's "Cleaner" -- on the job.
ABC News' Huma Khan and Sunlen Miller contributed to this report.