This kind of thing happens all the time.

It's usually more because the defence lays it on thick about how terrible the sentence will be for the poor criminal and the jury feels guilty and doesn't want it on their conscience.

My aunt did this in the first trial she was on. They made them feel really guilty for the young Asian rapist, how it would ruin his whole life to go to prison, so they found him innocent when they all thought he'd done it. They were all feeling smug about themselves and how they were so superior and wise for giving this young man a second chance. They sat there waiting for him to be relieved by the verdict and walk out to his freedom. No, he was put in handcuffs and lead away to serve time for all the other rapes he'd already been coincided of and they all felt pretty stupid.

Since then whenever my aunt has been on a jury she's always pushed for guilty because she can no long trust what she hears from the defence.

Really arguing a sentence will ruin a defendant's life should be banned. Either they are guilty or not and punishment is suppose to upset a law breaker's life.