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A shocking error from Coventry City keeper Luke Steele gave Gillingham the lead in a lively first half.
Steele let Darren Byfield's speculative 25-yarder through his fingers on 15 minutes and the ball squirmed into the goal.
Byfield should have met it 2-0 on 27 minutes after being set up by Mamady Sidibe but his shot was saved by Steele.
Sidibe was then denied by Steele six minutes later after a clever ball from Paul Smith.
Coventry made Gillingham pay for the missed chances in the 37th minute, when Dele Adebola headed home the equaliser from a Steve Staunton cross.