China's Olympic champion Ye Shiwen finished a shock fourth in Monday's 200m individual medley final, while teenager Ruta Meilutyte smashed the world record in the women's 100m breaststroke semi-finals.
The 17-year-old Ye had been the hot favourite for Monday's 200m IM final at swimming's world championships in Barcelona, but finished a full second behind the medallists as Hungary's Katinka Hosszu claimed gold.
Hosszu, 24, won by a considerable margin, clocking 2mins 07.92secs with Australia's Alicia Coutts second at 1.47sec back and Spain's Mireia Belmonte 1.53 behind, but defending champion Ye was a noticeable absentee on the dais.
Ye finished more than a second behind the medalists, which surprised Hosszu.
"I'm sure she will get back up there soon, she's a great swimmer and I'm sure it is really tough to get back after an Olympic gold medal," said Hosszu.
Ye has her next chance at an individual medal in Sunday's 400m IM, in which she set the world record at the Olympics last year in controversial fashion by swimming the last leg faster than Ryan Lochte did when he won the men's Olympic title.
Lithuania's teenage star Meilutyte, who pulled off a shock win in the Olympic 100m breaststroke final last year, punched the water in delight as she took a tenth of a second off the world record to blast her way into Tuesday's final.
"The world record was always a dream for me, but yes, obviously a gold medal at the world championships would be like the cherry on top of the ice cream," said the 16-year-old.
Meilutyte broke the four-year-old mark of 1min 04.45secs, set by Jessica Hardy of the USA, who will get the chance to reclaim her record in the final.
Swedish teenager Sarah Sjostrom regained her women's 100m butterfly title, while defending champion and world record-holder Dana Vollmer could only take bronze.
Sjostrom, who won the 2009 title aged just 15, hit the wall at 56.53secs, ahead of Coutts and Vollmer, America's Olympic champion who set the world record in winning the Olympic crown.
Coutts claimed two silver medals in Monday night's session, leaving her with three silvers in two days after anchoring the Australian team in Sunday's 4x100m freestyle relay.
"It has been a big two days for me and I am really happy because now I have two days off," said Coutts, who won five medals at the Olympics, Australia's best haul in the pool in London.
There was more good news for the Antipodeans as Christian Sprenger gained revenge for his defeat in last year's Olympic final when he won the men's 100m breaststroke gold.
South Africa's Olympic champion Cameron van der Burgh, who edged Sprenger into second in the Olympic final, was second and Brazil's Felipe Lima third.
Exactly a year to the day after the Olympic final, the 27-year-old Sprenger took his revenge as he overhauled the South African, the halfway leader, on the return lap
In Monday's other final, Brazil's Cesar Cielo retained his men's 50m butterfly world title, just ten months after surgery on both knees.
Cielo claimed gold in a winning time of 23.01secs with Eugene Godsoe of the USA just four hundredths of a second behind claiming silver and France's Frederick Bousquet at 0.10sec back to win bronze.
US teen star Missy Franklin, who is bidding to capture eight gold medals in Barcelona, moved safely into Tuesday's 100m backstroke final as the fastest qualifier.
Lochte, the defending world title holder, and France's Olympic champion Yannick Agnel both eased into Tuesday's 200m freestyle final.
Russia's Danila Izotov swam the fastest time in Monday's semi-finals, with Lochte just 0.22 of a second behind, while Japanese teenager Kosuke Hagino was third fastest.
The 18-year-old faces double finals on Tuesday having also qualified for the 100m backstroke and has entered five events with the 200 and 200IM still to come.