Via Gulliver, one of the pilots on a Toronto to Zurich flight in January 2011 took evasive action to avoid...a planet:
The FO [first officer] had rested for 75 minutes but reported not feeling altogether well. Coincidentally, an opposite–direction United States Air Force Boeing C–17 at 34 000 feet appeared as a traffic alert and collision avoidance system (TCAS) target on the navigational display (ND). The captain apprised the FO of this traffic.
Over the next minute or so, the captain adjusted the map scale on the ND in order to view the TCAS target 5 and occasionally looked out the forward windscreen to acquire the aircraft visually. The FO initially mistook the planet Venus for an aircraft but the captain advised again that the target was at the 12 o'clock position and 1000 feet below. The captain of ACA878 and the oncoming aircraft crew flashed their landing lights. The FO continued to scan visually for the aircraft. When the FO saw the oncoming aircraft, the FO interpreted its position as being above and descending towards them. The FO reacted to the perceived imminent collision by pushing forward on the control column. The captain, who was monitoring TCAS target on the ND, observed the control column moving forward and the altimeter beginning to show a decrease in altitude. The captain immediately disconnected the autopilot and pulled back on the control column to regain altitude. It was at this time the oncoming aircraft passed beneath ACA878. The TCAS did not produce a traffic or resolution advisory.
The airplane experienced -0.5 g to +2.0 g vertical accelerations, which caused several passengers sleeping across seats in Economy Class to smash into the overhead bins in one direction and into the armrests in another. (Always wear your seatbelts, folks.) In all, 14 passengers and two flight attendants got hurt.
The Canadian Transportation Safety Board report continues, analyzing the pilot's home life (he recently had children, and now doesn't get as much sleep), and the effects of overnight North America to Europe flights, noting "these types of flights are characterized by long periods of darkness with few operational demands while mid–Atlantic, creating inherently soporific conditions."
For an incident in which no one was seriously injured, the CTSB has prepared a thorough report with multiple points of action. It's worth reading, especially if you wonder why I prefer taking American's 9am flight to London instead of the 11pm flight.