Rookie mistakes cost 49ers a defensive TD on Ambry Thomas fumble return

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The San Francisco 49ers appeared to punctuate a dominant effort with a defensive touchdown against the Jacksonville Jaguars on Sunday. 

But a preemptive sideline celebration cost cornerback Ambry Thomas the score on a would-be 93-yard fumble return. 

With Jacksonville threatening to cut into San Francisco's 20-3 lead, Thomas stripped Jaguars receiver Christian Kirk of the ball after a red-zone catch on a throw from Trevor Lawrence. He picked it up and ran untouched down the sideline for what looked like a score. 

But the touchdown was nullified. 

Thomas was allowed to run unencumbered in part because players from both teams thought the play was over after he recovered the fumble. But he wasn't touched down on the recovery. Players from both teams didn't realize the ball was live until he took off sprinting down the sideline. This includes players on San Francisco's sideline. 

Rookie linebacker Dee Winters and rookie safety Ji'Ayir Brown were among a group of 49ers players and sideline staff who took to the field to celebrate Thomas' fumble recovery. They did so during the return, apparently thinking that the play was over. 

That's obviously against the rules. The 49ers were flagged for unsportsmanlike conduct, nullifying the bulk of the return including the touchdown and establishing possession of the ball for the 49ers at their own 19-yard line. 

Fortunately for the 49ers, their offense made good on the turnover. They capped a seven-play, 81-yard drive with a 23-yard Deebo Samuel touchdown to extend their lead to 27-3. 

Even when things went wrong on Sunday for the 49ers, they ended up going right.