Swansea was humbled by fourth-tier Oxford for the first big shock of the FA Cup third round, while Tottenham needed an 89th-minute penalty from Harry Kane to salvage a 2-2 draw against Leicester on Sunday.
Chelsea endured third-round humiliation at the hands of lower-league opposition in Bradford last season but had no problems 12 months on, with a full-strength team beating third-tier Scunthorpe 2-0 through goals by Diego Costa and Ruben Loftus-Cheek.
The world's oldest knockout competition was short of upsets on Saturday and an injury-hit Liverpool scrambled to secure a nervy draw at Exeter on Friday. But Oxford caused a surprise with a 3-2 win over Swansea, which can now concentrate on its relegation fight in the Premier League.
Fifty-four places separated Swansea and Oxford in the English football pyramid and Ecuador winger Jefferson Montero gave an indication of the supposed difference in class by backheeling the visitors into the lead in the 23rd minute.
However, Oxford scored three goals in 14 minutes either side of the halftime break, and Bafetimbi Gomis' 66th-minute reply proved only a consolation for Swansea.
Swansea came unstuck after making 10 changes for the game.
"All credit to Oxford, they were more energetic than us and passed the ball better," Swansea manager Alan Curtis said. "We made changes but we still had lots of experience, but Oxford thoroughly deserved to beat us."
Oxford won the League Cup in 1986 and was most recently in England's top division in 1988, before dropping down the leagues and tumbling into the non-league in 2006.
"I've got to be delighted beating a Premier League side — but not just that, it's the way we did it," Oxford manager Michael Appleton said.
Four Premier League teams were eliminated Saturday, but only after losing to fellow top-flight sides. It looked like Spurs would be joining them, only to earn a reprieve when Leicester winger Nathen Dyer flicked out his hand to divert the ball away.
Kane, on as a second-half substitute, converted the penalty to secure a replay and a third game in 10 days against Leicester. They meet in the league on Wednesday, too.
Leicester manager Claudio Ranieri rested many of the key players who are helping the team make a surprise run at the Premier League title but the stand-ins delivered, threatening to eliminate Spurs for the second straight season.
After Christian Eriksen put Tottenham ahead, Poland defender Marcin Wasilewski equalized by heading in a corner before Japan striker Shinji Okazaki — a halftime substitute by Ranieri — dribbled his way into the area and scored at the second attempt in the 48th.
Chelsea remained unbeaten under Guus Hiddink, who took over until the end of the season after Jose Mourinho's firing last month, but it wasn't a convincing performance by the seven-time FA Cup winners.
Under pressure from two defenders, Costa turned in Branislav Ivanovic's right-wing cross for the 13th-minute opener. Loftus-Cheek, a substitute, ended Scunthorpe's resistance in the 68th.
Hiddink guided Chelsea to the FA Cup title in 2009, also as an interim manager.
The draw for the fourth round takes place Monday.