Luton Town players were furious as they believed Tammy Abraham's first goal for Chelsea against them should not have counted as there were two ball son the pitch.
Abraham swept home a fine strike from inside the penalty area after 11 minutes after picking up a deflected cross from Timo Werner down the right hand side.
Just seconds before the goal, Hakim Ziyech took a quick throw-in for Chelsea while there was a separate ball on the pitch which distracted some of the Luton players.
The laws of the game say the second ball must interfere with the play, but even though the balls were about 15 yards apart, BBC Sport pundit Danny Murphy believes the goal should have been chalked off.
Murphy claimed at half-time: 'It's an interference if you're a player on the pitch watching the ball that's on the pitch. I think they have every reason to be upset.
'If you're a centre-half or central midfielder looking at the ball that's on the pitch thinking "what are they going to do with it?" while the other ball five or ten yards away is on the pitch being played behind your back players. I don't think it should have stood.'
Fellow BBC Sport pundit and former Chelsea striker Jimmy Floyd Hasselbaink added: 'I think Luton is a little bit unlucky.
'Ziyech can't do anything about it because he doesn't expect that ball to come back in But the referee sees it so I don't think it should have stood.'
Abraham then doubled the lead six minutes later to put Frank Lampard's Chelsea very much in control of their fourth round tie at Stamford Bridge.
However, Blues goalkeeper Kepa Arrizabalaga made a horrendous error to allow Jordan Clark to score.
The Spaniard allowed the Luton player's simple shot to go underneath his body to give the Championship side hope for the second half.