The first game of rugby played at the Olympic Stadium threatened to turn into something of a wash-out when the on-pitch sprinklers came on.
The stadium will host five games in the World Cup in September and October.
Samoa were playing the Barbarians in a World Cup warm-up match when the sprinklers came on unexpectedly in the first half.
The game eventually finished in a 27-24 defeat for Samoa, who had lock Kane Thompson sent off for punching.
Samoa's Thompson faces an anxious wait to see whether he will be banned for the World Cup, which starts on 18 September when England play Fiji at Twickenham.
He was sent off in the 16th minute for punching Barbarians hooker Saia Fainga'a, who was sin-binned for his part in the incident.
The Barbarians scored tries through former All Black Adam Thomson (two) and Australians Liam Gill and Ben Tapuai (two), with Samoa replying with scores from Anthony Perenise, Ken Pisi and Jack Lam.