Arsene Wenger accused referee Neil Swarbrick of making a "scandalous" call as a dubious second-half penalty helped Watford rally to beat Arsenal 2-1 at Vicarage Road on Saturday.
Club captain Per Mertesacker celebrated his first Premier League appearance in 532 days with a thumping header to give the visitors a first-half lead.
However, after Alex Iwobi and Mesut Ozil failed to convert glorious opportunities to double Arsenal's lead, Swarbrick judged Hector Bellerin had fouled Richarlison inside the area, enabling substitute Troy Deeney to score an equaliser from the spot in the 71st minute.
Watford went on to grab an injury-time winner too, as Tom Cleverley popped up in the right place to slam home a loose ball and hand Arsenal their first defeat in eight games.