Rebecca Balding first appeared in season 1 episode 7 The Fourth Sister as aunt Jackie. In this episode she plays Phoebe's future boss Elise.
This is the first time we are introduced to the Bay Mirror, which is where Phoebe works the remainder of the show.
The title is based upon the expression "the fifth wheel" meaning a useless extra item on a four wheel vehicle.
You can see Holly Marie Combs' wrist tattoos when she answers the phone while at P3. They don't seem to be concerned about covering the actresses tattoos anymore, which was mostly poorly done anyway.
After Piper freezes the street to stop a taxi from running over the innocent, Karen Young, we can clearly see Karen moving just before Pheobe tackles her to the ground. Only good witches don't freeze, but Karen Young is an innocent, not a good witch.