The crash was reported shortly after 2 p.m. on Alameda De Las Pulgas at Gordon Avenue, CHP Officer John Short said.
An 80-year-old Atherton woman had been trying to park when she for some reason hit the accelerator and crashed through the front window of Avanti Pizza, CHP Officer Curtis Glace said.
A 26-year-old Newark woman who was working as a cashier at the restaurant was pinned between a table and a wall.
She was extricated and in stable condition when she was transported to Stanford Hospital, Glace said.
The driver was not hurt, he said.
Glace said that investigators are still trying to determine why the driver hit the accelerator, but they do not believe it was intentional, and there was no indication that alcohol or drugs were involved.
"It's possible that she just hit the accelerator instead of the brake," Glace said.
A contractor who did not witness the crash was boarding up the fagade of the pizzeria, where tables, stools, and broken glass lay on the floor inside.
An employee at neighboring Country Club Cleaners said she didn't see the accident but heard a loud crash when it happened.