Legia Warsaw ultras defied a UEFA ban at the Europa Conference League match against Molde on Thursday night.
The ultras section was closed for the knockout round play-off second leg following fan trouble before the tie against AZ Alkmaar.
Only children up to the age of 14 were allowed in the home end and 'this time you won UEFA' banner was placed behind rows of empty seats before kick-off.
But it proved to be a false flag as the ultras had simply moved to another part of the stadium for the game.
And they unfurled a 'surprise motherf*****s!' banner and let off fireworks as the teams lined-up at the Stadion Wojska Polskiego.
Legia Warsaw fans unveiled an offensive banner at Europa Conference League tie with Molde
On the pitch, Legia were beaten 3-0 on the night as Molde completed a 6-2 victory on aggregate.
The Polish club could now face further punishment from UEFA.
Legia fans have been banned from attending their club's next five European away games following the violent scenes that marred last month's Europa Conference League clash with Aston Villa.
The Polish club were also fined £100,000 (£86,000) and ordered to pay for the damage done to Villa Park during the incidents that took place before and during the tie on November 30.
Police charged 46 men after Legia Warsaw fans clashed with police, with missiles and flares thrown at officers as supporters were held in a coach park near the ground before they were due to enter the stadium.
West Midlands Police said four officers were injured and, although the game started on time, no Legia fans were allowed in ahead of kick-off.
As well as the violent clashes, Villa stewards also removed several people believed to be away supporters from the home stands while footage on social media appeared to show objects being thrown into the stadium from outside.
Villa made an official complaint to UEFA regarding the behaviour of the Legia supporters after the 2-1 win.
Legia Warsaw ultras clashed with fans and ranks of police outside Villa Park last month
An officer was set on fire from a flare and five officers were injured before the Euro tie
Legia Warsaw were fined 50,000 euros (£42,000) in 2017 for a fan banner criticising UEFA.
The club were also ordered to close the north stand of its stadium, where the trademark pre-match displays are made by fans.
UEFA intervened in response to Legia fans displaying a giant banner of a UEFA badge with a pig and the slogan 'And the 35,000 fine goes to...' at a Europa League game.
That related to UEFA's fine for displaying a political banner commemorating the Warsaw Uprising in 1944 before a Champions League qualifying game.
Legia also hosted Real Madrid in an empty stadium after fan violence.