A fan almost got onto the pitch to confront Chelsea players during Monday night's Premier League match at West Ham.
The incident occurred after Eden Hazard had fired the Blues into a 25th-minute lead.
The fan - who emerged from the home end - climbed over the advertising hoardings and got to within a few feet of Hazard and his team-mates as they celebrated, before he was tackled by stewards and ushered away.
The EFL Cup meeting between the teams in October was marred by crowd trouble when rival supporters threw coins, bottles and seats at each other.
While there was no sign of a repeat of those ugly scenes, the security breach will be another major concern to those in charge at the London Stadium.
Another fan was ejected from in front of where the Chelsea supporters were housed as the game approached stoppage time, but the match otherwise passed off peacefully.
West Ham had implemented a robust five-point security plan since Chelsea's last visit, when seven fans were arrested on the night.
The segregation area between rival supporters was widened, with a physical barrier also put in place, along with a 10-metre 'sterile' area as well as new measures to keep rival fans apart after the game.
Stewards have been issued with handheld video cameras to help identify troublemakers, while a 'no tolerance' banning order policy is in place for anyone found guilty of offences ranging from abusive and offensive language to missile throwing.
So far this season West Ham have banned 97 fans.
The measures seemed to have had the desired affect as, barring those two isolated incidents, there was no hint of crowd trouble.
Chelsea eventually ran out 2-1 winners, with Diego Costa doubling their lead before Manuel Lanzini netted a consolation in stoppage time.