Eric Lichaj's first-half double put Forest on their way before penalties from Ben Brereton and Kieran Dowell - the second in controversial circumstances - helped the Championship outfit over the line.
Per Mertesacker and Danny Welbeck got on the scoresheet for Arsenal, but Arsene Wenger watched from the stands as the club went out of this competition at the last-64 stage for the first time in over two decades.
Find out how all of the action unfolded in the East Midlands courtesy of Sports Mole's minute-by-minute updates below.
NOTTINGHAM FOREST SUBSTITUTES: Henderson, Mills, McKay, Bouchalakis, Walker, Cummings, Vellios
ARSENAL SUBSTITUTES: Macey, Chambers, Osei-Tutu, Dasilva, Reine-Adelaide, Akpom, Nketiah
NOTTINGHAM FOREST XI: Smith, Lichaj, Mancienne, Traore, Osborn, Cash, Clough, Brereton, Dowell, Vaughan, Worrall
ARSENAL XI: Ospina; Debuchy, Mertesacker, Holding; Nelson, Willock, Elneny, Maitland-Niles; Walcott, Iwobi; Welbeck
If you would like to read our on-the-whistle match report, you can do so by clicking here. Goodbye for now.