Beyonce is actually really good at lipping. She goes the extra mile to convince everyone she's singing live. She uses a handheld mic and she re-records her songs with little speaking bits, breaks, etc.
I always notice when she's not singing for real. She always makes little mistakes like that, and for some reason I always catch her in the act. I don't care that she's doing it, but I just felt like bringing it to everyone's attention. Nobody ever calls her on it and it's rather annoying.