The Emirates' atmosphere was hailed as among the best in the stadium's history for Arsenal's Champions League quarter-final at home to Real Madrid, so there were high expectations for Tuesday's semi-final against Paris St-Germain.
A fourth-minute goal for the away side punctured that, but Arsenal's tifo display also fell flat.
On social media, rival fans mocked the giant red flag with a white cannon, while some Gunners supporters also expressed their disappointment at what was unveiled.
It was the club's first Champions League semi-final appearance since 2009, so fans wanted something befitting the occasion.