That's normal. I've done well with live betting when I feel like action is easily pointing one way after watching a while. It most certainly can back fire, but you can sometimes find a better number, 4, 8, 11, etc. after some live action. However, you have to be careful with the juice. You'll get a lot of those at a higher rate than the original lines.
I live bet USC on Saturday after the 4th drive from Utah as their offense looked putrid and had heard they had barely practiced before that game, so felt comfortable at that point with USC -3. However, I believe it was -117 or something like that, where normal line is typically around -110, depending on where you bet.