Henderson had seen a spot-kick saved in the 0-0 draw at leaders Bristol City on Tuesday night and was this time denied by Potters goalkeeper Carlo Nash, after referee Rob Styles had harshly penalised captain Andy Griffin.
Watford had earlier been were reduced to 10 men at Vicarage Road, captain John Eustace shown a red card against his old club for a clumsy challenge on Richard Cresswell.
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