Character mistake: The American Flag is hung improperly on the wall. The Flag's union (stars) should be at the top, to the observer's left, but it's to the observer's right. (00:06:38)
Continuity mistake: At end of the episode, when Sherlock leaves Detective Bell outside the 11th Precinct, Bell had a book and an envelope of which Bell puts the envelope in his pocket leaving the book, the next shot shows Bell turn and throw the envelope in the trash and the book disappears. (00:39:15)
Character mistake: Sherlock and Joan are looking at video footage of a restaurant called "La Porte Blanc", which is incorrect French. The right way is "La Porte Blanche."
Answer: I'm the same person who asked this question. I discovered it was Molly Price whom I recognized from Third Watch.
It should be noted Donna was Jim Fowkes‘ secretary. Donna killed Peter.