B737-800 first officer reported failing to ensure parking brake was set after taxi into gate.

Attributes
ACN	1653423
Time
Date	201906
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZ.Airport
State Reference	US
Environment
Light	Daylight
Aircraft 1
Make Model Name	B737-800
Operating Under FAR Part	Part 121
Flight Phase	Parked
Flight Plan	IFR
Person 1
Function	Pilot Not Flying First Officer
Qualification	Flight Crew Instrument Flight Crew Air Transport Pilot (ATP) Flight Crew Multiengine
Experience	Flight Crew Last 90 Days 465
Events
Anomaly	Deviation - Procedural Published Material / Policy Ground Event / Encounter Loss Of Aircraft Control

Narrative:

We pulled in to [the gate]. The captain began his shutdown flow and either forgot to set the parking brake or somehow it was released. I went to grab the checklist and out of the corner of my eye; I noticed we were moving backwards. The captain immediately applied the brakes and they connected the tow-bar and towed us back to the line after we made sure everyone was back in their seats. It was a really long day with a weather diversion and almost timing out. We were eager to get the passengers on their way after a very long flight. We had started the checklist and I did not visually verify that the parking brake was set when I was completing the parking checklist.

Original NASA ASRS Text

Title: B737-800 First Officer reported failing to ensure parking brake was set after taxi into gate.

Narrative: We pulled in to [the gate]. The Captain began his shutdown flow and either forgot to set the parking brake or somehow it was released. I went to grab the checklist and out of the corner of my eye; I noticed we were moving backwards. The Captain immediately applied the brakes and they connected the tow-bar and towed us back to the line after we made sure everyone was back in their seats. It was a really long day with a weather diversion and almost timing out. We were eager to get the passengers on their way after a very long flight. We had started the checklist and I did not visually verify that the parking brake was set when I was completing the parking checklist.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.