Here is an article submitted by Steven Lynch for cricinfo on World Cup games that produced unexpected results. According to this article Sri Lankans involve in three of these 11 matches.
Sri Lanka beat Australia, 1996
The 1996 World Cup was probably the most evenly contested, with no clear early favourite emerging. West Indies beat the fancied South Africans in the quarter-final, and looked all set to make the final until a spectacular late collapse against Australia. That set up the climax, against surprise packets Sri Lanka, whose electrifying opening pair of Sanath Jayasuriya and Romesh Kaluwitharana had added a new dimension to 50-over cricket. It was two older Sri Lankan hands, though, who controlled the final in Lahore: Arjuna Ranatunga ignored local advice to bat first, then Aravinda de Silva took three wickets with his flattish offspin to restrict the Aussies to 241. Ranatunga fielded first because he had noticed that evening dew made gripping the ball a problem for the bowlers under the floodlights, and he and de Silva guided their side home as Shane Warne and friends found the slippery ball hard to control.
Kenya beat Sri Lanka, 2003
Nairobi had never seen cricket scenes like it: "Kenya chased like lion cubs, backed up in gangs and jigged after every wicket," reported Wisden. "Sri Lanka, by contrast, simply moped: ‘The worst game of my career,’ admitted Sanath Jayasuriya." Victories over Bangladesh and Zimbabwe then propelled Kenya into the World Cup semi-finals for the only time.
Sri Lanka beat India, 1979
If Bangladesh’s victory over Pakistan in 1999 helped them gain Test status, this one at Old Trafford 20 years earlier did the same for Sri Lanka. India had already lost heavily to West Indies and New Zealand, and now went down by 47 runs, never really threatening Sri Lanka’s workmanlike total of 238. After this, India’s only World Cup victory remained one over lowly East Africa in 1975: they were to put things right with a vengeance in 1983. By then, Sri Lanka were a Test-playing nation too.