The original ending at the restaurant was supposed to be a special effect to complement the earlier mirror shot. The scene was supposed to transition to the alternate realm as the camera dollied by the case. A crew reflection made the footage unusable and digital removal was too expensive.
The name on the clinic sign is Henry Pratt, the real name of actor Boris Karloff.
The opening scene was originally scripted to take place in a grocery store, which would allow for a cut from the aisle to the counter where the main character would already be in the middle of purchasing items. The store closed permanently just two weeks before principal photography, forcing the production to move to a local bistro.
The clinic sign is actually a rigid poster board with the symbol and letters taped on. It's attached to stakes driven into the dirt. It was shot at a low angle to make the sign seem larger than it is.
The exterior of the psychiatric clinic was actually Pringle Hall Park in Salem, Oregon.