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Butler felt he had been caught in the face by Rovers' Andy Robinson in the 62nd minute and when referee Mark Brown ignored it, the defender brought down the midfielder on the edge of the box.
It meant a second defeat in a week without scoring for Walsall who are hovering just above the relegation zone.
Their manager Dean Smith said: "The lads feel aggrieved because they felt we did enough to win the game. A decision has gone against us and cost us points.
"We felt we should have had a free-kick. The lad elbowed Andy in the face and the referee allowed the game to go on. But Andy should have kept his discipline. He gave away a free-kick and they scored."
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