Millwall surrendered a two-goal lead and had right back Alan Dunne sent off for a second bookable offence as they were crushed by a resurgent Bournemouth.
Bournemouth centre-half Steve Cook needed treatment after celebrating his team's fourth goal when it appeared a missile had been thrown at him by Millwall fans.
The offending object was handed to referee Lee Collins before play resumed, and a large police presence moved into the Millwall section of the Goldsands Stadium.
For Millwall it was their second heavy defeat in the space of four days, after being beaten 4-0 at Birmingham.
Millwall appeared to be coasting at 2-0 before Bournemouth came back after the break with three goals of their own in nine minutes and they recorded their fifth win of the season.
Millwall went ahead after only six minutes when Martyn Waghorn slotted home his second goal of the season from Liam Trotter's defence-splitting pass.
Bournemouth were still reeling from that setback when they conceded another four minutes later when another big gap appeared in their defence.
This time it was Waghorn who was the provider, with a pass which Trotter neatly lifted over on-loan goalkeeper Stephen Henderson.
Millwall had three players booked in an eight-minute spell in the first half, Alan Dunne and Shaun Derry for fouls and former Bournemouth left-back Scott Malone for a transgression at a free-kick.
Millwall almost added a third in the 26th minute with a left-footed shot from Lee Martin which flew narrowly wide as Henderson dived to his right.
Bournemouth improved as the first half wore on and pulled a goal back in the 43rd minute.
Scotland Under-21 winger Ryan Fraser took a pass from Lewis Grabban and curled a right foot shot just inside the post.
Bournemouth almost snatched an equaliser in first-half injury time when Jack Collison, like Henderson on loan from West Ham, had a fierce shot turned over the bar by Millwall goalkeeper David Forde.
Bournemouth stormed into the second half and scored three goals in nine action-packed minutes.
The hosts equalised in the 50th minute when Steve Cook rose highest to head Ian Harte's corner in off the bar.
Harte went close when he lined up a trademark free-kick, but his effort crashed against the bar from 25 yards in the 54th minute.
The comeback was complete moments later when Tokelo Rantie squared the ball to Harry Arter who smashed home from just outside the box.
Bournemouth were then awarded a penalty in the 59th minute when Dunne was given a second yellow card for a deliberate handball in the box.
Grabban converted from the spot and celebrated in front of the travelling fans and it appeared a missile was thrown from that area of the ground which Cook.
Bournemouth completed their revival with a fifth goal in time added on at the end, when Fraser was brought down by substitute Paul Connolly and Brett Pitman smashed home the second spot-kick of the match.