The Senegal international was a surprise choice by Frank Lampard before kick-off, making his first start for Chelsea since the World Cup.
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But Mendy has done little to persuade that he can hold down the No. 1 spot after a terrible error that allowed Taiwo Awoniyi to score.
Forest left-back Renan Lodi sent in a fine cross after Mateo Kovacic lost the ball while Awoniyi found himself between both the centre-backs.
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And Mendy made the inexplicable decision to come rushing out to try punch the ball away but got nowhere near.
With the goalkeeper in no man's land, the Forest striker had the simple task of nodding into an empty net - giving Steve Cooper's men a vital lead in the battle to stay up.
Mendy was left crumpled on the floor with his head buried into the turf, aware of the clanger he just made.
It gives Lampard another miserable start to the afternoon, having believed that Chelsea had turned a corner last week.
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A 3-1 win over Bournemouth gave him his first win since taking the Blues job until the end of the season in April.
It had been six defeats in a row before that and Chelsea might be returning to their dire form after the anomaly of three points against the Cherries.
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As for Forest, it gives them a potentially crucial lead as they look to beat the drop this season.
After Leeds drew against Newcastle on Saturday afternoon, it put them a point behind the City Ground outfit but still in the relegation zone.
But a win at Stamford Bridge would put Forest five points clear of the bottom three with two games left, giving them a major boost in their bid to survive.
More will follow.
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