ZDV D-Side Controller conducting OJT; described a airspace entry event without an handoff because of an inadvertent flight data block removal action; noting extreme care must be used when using the 'slant OK' functionality.

Narrative

I was providing OJT (On Job Training) on Sector 18 D-side at Denver Center. The supervisor informed us that a Flight Data Specialist had accidentally removed an active flight plan and he believed it to be on an aircraft in our sector. I immediately scanned the sector and observed an uncorrelated beacon in the middle of the sector squawking and at FL390 eastbound. I alerted the R-side to it as the Supervisor told us that the call sign was Air Carrier X. The R-side called for Air Carrier X on the frequency and the pilot responded. The R-side asked him to IDENT it was observed on the target in question. The target was about 4 minutes from the Kansas City Center boundary so I called the ZKC with a point out and a story as to what was going on with the flight plan. I told him that we (the Controllers at ZDV) would get the flight plan in and to expect a hand off after that was accomplished. The R-side queried the pilot for the details of his flight plan as no flight plan information was available to us (nothing in HOST or URET). We entered the flight plan information; started a hand off to ZKC (the aircraft was about 30 miles inside their airspace by now); and shipped him. The next day I asked for further details of what happened in our flight data department. The flight data specialist was trying to remove a different flight plan (an inactive one) and made a typo; then noticed the error message but continued removing the flight plan via the 'slant OK' feature. He then realized that he'd made a mistake and promptly got on the phone to our Area and informed the supervisor of the situation. Recommendation; removing a flight plan using the 'slant OK' feature has always been taught to be a dangerous keyboard entry. You'd better triple-check before doing it because you're probably doing something wrong if you have to do it. I think a refresher for everyone; warning of the consequences might be a good idea.