West Ham boss David Moyes was left angered after their 1-1 draw with Europa League quarterfinal opponents Lyon.
A historic match for the Hammers ended on a sour note after a pitch invader - the second of the game - jumped out of the home end with 10-man West Ham actually mounting a rare attack.
"I was baffled," said Moyes. "This is a really good football club doing so many good things.
"But those things tonight didn't help, we were on a counter-attack with an opportunity to score."
There was also another incident on the night when a different supporter invaded the pitch. The development came just a week after Forest Green owner Dale Vince boasted paying people to invade and disrupt Premier League games.
Nevertheless, a draw keeps West Ham's dream of a place in the semi-finals, against either Barcelona or Eintracht Frankfurt, well and truly alive.