With stadium security asleep at the wheel, Ryan Crotty upended one of the intruders while forwards coach Dave Hewett also got in the action with a fine hit on another. The prolonged stoppage quelled any momentum the Crusaders had accrued but the visitors had only themselves to blame for earning nothing more than a bonus point.
"We had the ball and we coughed it up at vital times, and at key moments we couldn't maintain pressure," said coach Todd Blackadder. "We let ourselves down and we let them off the hook."
The Force have been something of a bogey side for the Crusaders and, coming into tonight, the seven-time champions had won only once from four matches in Perth. But Blackadder said there were no suggestions his side had underestimated the Western Australians.
"We actually prepared really well and we have no excuses. We're not going to say that we took them lightly or anything - we certainly didn't.
"We knew this was a big game for us, it was crucial and we didn't play that well. They obviously played better than us and we just made too many basic errors."
The Crusaders were perhaps lucky to go into halftime trailing only 16-8, given the disjointed nature of their attack. Luke Romano finished off a marauding run from Corey Flynn down the left flank to cancel out Sam Norton-Knight's early effort for the Force, while former Hurricane Jayden Hayward banged over three penalties for the hosts.
Romano did have another try ruled out by the TMO after a forward pass in the build-up, but Blackadder and co cut rather unhappy figures when they exited the coaches' box at halftime.
"Execution errors usually come from an average mindset so I wasn't pleased with the first half," Blackadder said.
The Crusaders' attack did apply themselves better to open the second spell, putting together a number of phases before eventually settling for a penalty from Tyler Bleyendaal to reduce the gap to five. Romano was then again denied by the TMO when he replays of his grounding were inconclusive, with the Crusaders forced to be content with another penalty.
The visitors huffed and puffed for the final 20 minutes but the resolute Force stood firm - aided by a few of the home fans.
"It was obviously a really key moment, too," Blackadder said of the final invasion. "We got a turnover and we would have counter-attacked and it was about five-on-one.
"It's poor - it shouldn't happen at this level. The crowd needs to be far more disciplined. It ruined a good game of footy."
The Crusaders will now be pleased to return to Christchurch and they will be equally happy to host the hapless Highlanders next Saturday.
"We need to get our house in order," Blackadder said. "We need to start playing some rugby where we can maintain the ball and stop making basic errors or else we're not going to be beating anyone."
