I spent ten weeks working with a female singer originally from a well known poppy group. She had a great voice, and singing the same songs live for 88 shows was hard work. So I got our sound op to take an iso feed from her headset mic and record it. We had 7 musicians in the pit, plus numerous stems being used for playback with click for various pad feeds - BVs, little guitar twiddly bits, the odd brass full section where our sax and trumpet wasn't full enough. We stuck her iso track in as a stem, and a few days later, she had a croaky voice. I revealed I'd recorded her, and asked if she fancied miming (lip-sync is just a term intended to prevent the dreaded MIMING word being used). She jumped at the chance, and said that in all the years she'd performed with the band, their mics had never been plugged into anything. She was perfect at miming - even the swallows you see in their throats was sync-perfect. She can sing pretty well, yet the miming became the way it went for every show and the audience were totally unaware. Of course it's cheating, but it does mean the end product is good. It just annoys me when some performers appear to be very weak live, and only good after the studio repair and enhance work has made their voice better than it really is.
