When the Charmed Ones are in P3, Piper freezes all the innocents, one of the warlocks throws a fireball at Paige, who then calls for it & throws it back at him. The flash from the fireball vanquishing the warlock causes one of the extras who is supposed to be frozen to squint & move his hand to cover his eyes.
The page in the Book of Shadows that describes muses has multiple grammatical errors, for example: 'who's' instead of 'whose', and 'whitelighter's' instead of 'whitelighters'. Although the sisters do add in new pages on occasion, the pre-existing pages in the book were created by magic, so there should be no such errors.
The Book of Shadows states that muses 'can only be seen by other angelic beings', so Leo (as a whitelighter) should be able to see them. But despite the sisters' muse Melody's assertion that 'we go way back', Leo never acknowledged her presence prior to the spell cast to make her visible, even when muses were being discussed directly.