Shadow Play - S4-E19
Other mistake: When Ezra takes Spencer's file from the school he takes it from section A-E. Neither of Spencer's names fit into that category.
Cat's Cradle - S4-E3
Continuity mistake: When Hannah is on the phone to Emily, Hannah pours a glass of fizzy drink (coke or whatever) and is about to drink from the glass, in the next shot she takes the can and walks away and the glass isn't seen again.
Continuity mistake: When Emily and Aria are contemplating climbing out of the window and down the trellis to get help, they hear Alison scream and the broken window slams down completely; the shot briefly changes then the window is shown half-way up.
