Continuity mistake: When Prue talks to Piper after stopping her phone conversation, she tells her that "the Sin Balls work a lot faster than [they] thought. Once infected, the victim only has a few hours before it drives them to total self-destruction." However, this is almost verbatim what was written in the Book of Shadows. It could be easily read by the viewing audience when the book entry was shown. So all three sisters should already know this.
Other mistake: At the start of the episode when Prue and Phoebe are walking down the street (just before they find the box), the person with the green jacket on (in the background) reaches into the homeless man's hat and just "scrapes" the bottom of it. Not putting any money in, or taking any money out - a completely pointless action.