Learned this lesson the hard way in my last session - I've been making rolls publically for a while, but I made a lethal combat roll against one of my player's level 1 PC while the rest of the party was too far away to help them.
I might have been able to find a way to not let them die from an enemy one-shotting them (there were many friendly NPCs nearby) but this would have really gummed up the pace of the game and story. Much easier to just fudge the roll and say they took almost lethal damage instead.
The trouble of course was the player saw the roll and so knew I'd fudged it. I don't want to set the precedent that I'm going to pull punches on them, but I definitely don't want anyone to die in their very first encounter (ever, not just in this campaign) not having made any real mistakes.
I'm now going to have to makes sure the party understands that was a one-time thing - and hide all my rolls going forward!