Borussia Dortmund fans disrupted their DFB-Pokal semi-final at Stuttgart on Tuesday evening by throwing tennis balls onto the pitch in protest at ticket prices.
Here, Press Association Sport looks at other unusual ways in which fans have made their voices heard.
More tennis balls
Dortmund supporters were not the first to throw tennis balls. Hull fans disrupted a League Cup tie against Bolton in 1998 in the same manner in protest at owner David Lloyd, a former tennis player and Great Britain Davis Cup captain. He sold the club later the same year. Fans of Sevilla, FC Basel and FC Luzern, meanwhile, have all thrown tennis balls in protest at kick-off times.
The atmosphere at Ewood Park had grown steadily more toxic during the 2011-12 season as Blackburn slipped towards relegation from the Premier League. Indian owners Venky's, a chicken meat processing firm, were the focus of most of the anger and at a crucial match against Wigan in May 2012 a chicken draped in a Blackburn flag was let loose on the pitch. Wigan went on to win the game, confirming Blackburn's demotion.
Luis Figo's transfer from Barcelona to Real Madrid in July 2000 was one of the most controversial in football history. Barca fans made their feelings known on Figo's first return to the Nou Camp, and things got even more heated the second time he went back in November 2002. The game was suspended for nearly 20 minutes because of the number of objects being thrown at Figo as he tried to take a corner, with the most famous missile being a pig's head.
Green and gold
Manchester United supporters' anger at the ownership of the Glazer family became symbolised by the wearing of green and gold scarves - the colours of Newton Heath, the club's first incarnation. The campaign received a boost in March 2011 when David Beckham left the pitch after a Champions League game against AC Milan wearing one of the scarves. FC United, currently in the National League North, were set up in 2005 by United fans opposed to the Glazer takeover.
Blackpool fans forced their final match of last season to be abandoned by staging a sit-in demonstration. With 48 minutes gone of the fixture against Huddersfield at Bloomfield Road, supporters protesting about the running of the already-relegated club poured onto the pitch and occupied the centre circle. Referee Mick Russell eventually called the game off after a delay of more than an hour.
One of the most bizarre protests came at Goodison Park in February 2012. Just before half-time in a match between Everton and Manchester City, John Foley ran onto the pitch and handcuffed himself to a goalpost in protest at his daughter's sacking by airline Ryanair. The company's boss, City fan Michael O'Leary, was reportedly in the crowd.