Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument
Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Other / Unknown
General Flight Cancelled / Delayed; General Maintenance Action
Human Factors; Procedure
While preparing for departure on my third leg of IOE; we had a crew consisting of myself; the Captain and two RFO's (Relief First Officers). (One additional; since I had not met the requirements to be an independent crew member). The Captain and I were in the seats having completed the pre-flight items; up to the before start checklist to the line; and were waiting for the Final Weight and Balance data.  A few moments later we received the chime for the Final Weight and Balance data; and began to review it. The Captain heard the door being closed and told the second RFO; who was in the cockpit; that he didn't want the doors closed yet. The second RFO asked if the Captain wanted him to go back and stop the RFO who was closing up. The Captain said yes. As the second RFO went back; I heard him say 'hold up.' The Captain and I went back to looking at the final when the second RFO stuck his head in the cockpit and said 'we just blew a slide'.A few moments later the RFO who was in the supernumerary compartment came up and said 'I can't believe I just did that.' He proceeded to say that it was totally his fault; there was no malfunction and he just made a mental error.At that point the Captain began contacting the appropriate people to start working the problem.
[Report narrative contained no additional information.]
Air carrier flight crew reported an inadvertent slide deployment of a door during preflight procedures.