Any Unknown or Unlisted Aircraft Manufacturer
Air Traffic Control Fully Certified
Situational Awareness
ATC Issue All Types; Airspace Violation All Types; Deviation / Discrepancy - Procedural Published Material / Policy
General None Reported / Taken
Human Factors; Procedure
I received a position relief briefing from the previous Controller. He had accidentally dropped the track of Aircraft X instead of the intended arrival; but didn't realize it. At the time I signed in to take the sector; the limited Data Block of Aircraft X was at the boundary leaving my airspace and I didn't notice it. I remember Aircraft X calling me later to ask for a higher altitude. I did not remember working him; so I pulled up his Flight Plan and ran a route line to see where he was. The route line came up in the middle of the adjacent sector's airspace; and I saw the call sign in the limited Data Block next to the route line; so I sent him to that sector's frequency (Sector 16). Since dropping a Data Block is so easy to do and can cause such an unsafe condition; it might help to make dropping Data Blocks have the same procedure as removing a flight plan; where a /OK entry is required. Perhaps many people just /OK without really double-checking; but it might save some people from making a mistake.
I was working Aircraft X. I had 2 aircraft on approach at BZN: Aircraft Y; a B757; followed by Air Carrier Z; a CRJ; both on the visual approach. BZN Tower called with a downtime on both aircraft just as Aircraft X was over BZN field and I accidentally dropped Aircraft X and Aircraft Y's tracks. A few moments later I looked back and noticed that Air Carrier Z's track was still there and dropped it. I remember thinking that I thought that I had already dropped the track and I should go back and recall my entries to see what I had actually typed but then I was distracted with another BZN departure and forgot to investigate. I began my position relief briefing and failed to notice the code a few miles from the boundary of Sector 16. When new Controller [came on] the aircraft was just crossing the boundary. I needed to follow through and pay better attention.
ZLC Controller described an airspace incursion event immediately after assuming the position.  Reporter noted that the previous Controller had unintentionally dropped a data tag causing the incident.