Date: 202008
Local time of day: 1201-1800
Location: ZZZ.ARTCC, US
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Communication Breakdown; Confusion; Other / Unknown; Situational Awareness
Anomaly: ATC Issue All Types; Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Environment - Non Weather Related; Human Factors
Primary problem: Human Factors

Narrative: We were in the climb to our last assigned altitude of FL300 and switched to ZZZ Center as we were passing through FL295. On the initial check in the FO [PF] incorrectly heard to continue climb to FL330 associated with our call sign. The PF pointed at the Altitude Window for FL330 to confirm; the CA [PM] wasn't 100% sure whether they had given us FL330. The PF immediately initiated a level off pressing the Vertical Speed 0 Button. We ended up leveling off at approximately 30;100 feet and in the level off ZZZ Center confirmed that they had not given us higher. Reset FL300 in the window and managed the descent back to our previously cleared altitude. The location of the event occurred IVO (in vicinity of) the ZZZZZ waypoint. Following the event ZZZ Center never mentioned anything about a loss of separation or about us being off our altitude by 100 feet. There were no aircraft in the immediate vicinity on TCAS. We landed safely at ZZZ1 and debriefed the event. As a crew we trapped an error utilizing SOP with the PF/PM both confirming the altitude set in the Altitude Window and prevented what could have been a major altitude deviation and possibly a loss of separation from another aircraft.Primary casual factors that contributed to the confusion and minor altitude deviation were as follows: PF was current; but not proficient after coming off a recent 4 month leave period; expectation bias of a continued climb since FL300 was not our planned final / expected final from ATC; and situational awareness to the fact that based on our position relative to our planned STAR / transition that a further climb wouldn't have made much sense under the circumstances. If there's any confusion to a clearance then immediately confirm with ATC and get clarification. If both crew members aren't 100% sure what they heard was correct; then the hairs on your neck should stick up until that's resolved. Additionally; following SOP prevented what could have been a significant altitude deviation. With many crew members coming off long leave periods; and stress due to COVID and possible furloughs; now more than ever it's important for all of us to follow SOP; do not rush; and back each other up if something doesn't look or sound right.




Synopsis: Air carrier First Officer reported an altitude deviation was the result of too much time away from work because of COVID-19 lack of flying.