Esher's 21-20 defeat at Sedgley Park brought their 100% record in National One to an end.
The Surrey side were leading 20-18 until the final kick of the game, a penalty, cost them the match and a first taste of defeat this season.
"It's been a reality check for some of these players who thought it was an easy ride," team manager Dave Page told BBC Surrey.
Esher still maintain a five point lead at the top of National One.
Page added: "I think it is complacency, I think we've got to look at selection and now there's a few players here who are going to be looking over their shoulder because they haven't been playing well.
"How much it meant to Sedgley Park just showed at the end at the whistle, it was as if they'd won the FA Cup final. It was just staggering, the joy in their faces, but I suppose as league leaders you're there to be taken."
Esher make the journey to promotion rivals, fourth place Cambridge, in their next match, on Saturday, 7 November.
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