Mauricio Pochettino criticised Southampton's focus after they nearly let a three-goal lead slip against Norwich City on Saturday.
Southampton looked to be cruising to victory when Jay Rodriguez made it 3-0 in the 72nd minute, adding to Morgan Schneiderlin's early opener and substitute Rickie Lambert's second-half strike.
But Johan Elmander and Robert Snodgrass scored inside the final five minutes to set up a tense finish.
It fell to youngster Sam Gallagher, introduced from the bench deep into stoppage time, to wrap up a 4-2 triumph with his first Premier League goal, but Pochettino was concerned that Norwich had been given a ray of hope.
"We were leading the game 3-0 and had it in control but we suffered a bit at the end," the head coach told BBC Sport.
"It's a great lesson and obviously in football nothing is finished until the final whistle. We lacked focus at the back and you can't take anything for granted in the Premier League.
"We were far superior to Norwich throughout the game and we were playing against a very good team.
"I didn't really see any meandering. Overall I thought the game was controlled. It is true that Lambert’s contribution was very good for the team, and also Sam Gallagher was also very good.
"We were satisfied with the 3-0, but we let our guard down. It is a lesson we have to learn."
The victory was only Southampton's second on home soil in nine top-flight matches and moved them up to eighth.comments