From A Wiki of Ice and Fire
|Theories may be removed if ...|
Jaqen H'ghar changes his face and name after he leaves Arya.  The description of his new face matches that of the Alchemist that meets with Pate in Oldtown. Pate later meets Samwell Tarly, but at the end of his Prologue chapter he appeared to have been murdered by the Alchemist. It's possible the Alchemist stole his identity.
- Jaqen H'ghar's new face is similarly described to that of the Alchemist.
- It is unclear whether Pate dies at the end of his chapter or merely passes out, though all other prologue characters either die immediately at, or shortly after, the end to their chapters.
- It is very unlikely the Pate Sam meets when he reaches Oldtown is the same Pate in the prologue of "A Feast for Crows." The Pate in the prologue hates being connected to Spotted Pate the pig boy  while in the final chapter of A Feast For Crows he introduces himself as "Pate like the pig boy". 
- Jaqen H'ghar can change the appearance of his face at will, and would easily be able to copy Pate's.
- It's quite strange for a Faceless Man to be in the Black Cells of the Red Keep. He would not have left Braavos without a distinct purpose.
- Who hired Jaqen H'ghar and who is he meant to kill?
- How did this person pay the House of Black and White for the assassination?
- How and why did "Jaqen" end up in the black cells?