Revealing mistake: The cake Mildred lifts is obviously a solid prop as the "frosting" does not smear her dress when she holds it close to her body.
Continuity mistake: When Mildred Pierce is running out of Monty Beragon's cottage to go swimming in the ocean, she's just beginning to put her swim cap on. The shot changes, less than a second or two has passed and now all her hair Is under the cap, it's buckled, and her arms are down as she enters the surf.