Sporting Lisbon produced a stunning late victory over Tottenham Hotspur in the capital on Tuesday.
Spurs wasted a number of good chances and they were ultimately punished when Sporting scored twice in stoppage time.
The first came in the 91st minute Paulinho headed in from Pedro Goncalves' left-wing corner.
Ruben Amorim's side put the game beyond doubt two minutes later another substitute, Arthur Gomes, hit a fine, low strike to make it 2-0.
It is Tottenham's first loss of the season and they are now second behind Sporting in Group D.
Speaking after the game, Conte said: "The Champions League level is high. You know that you have to reduce the mistakes because in the end you can pay.
"Tonight we paid a lot. Now we are talking about a loss, a loss against a team that showed they are used to playing this competition."
