Continuity mistake: When Anna and her mum are at the rehearsal dinner, Jamie Lee Curtis (in Anna's body) cries. In the first shot, her first tear has rolled down to her lip, but in the next, it is around the same level as her nose.
Continuity mistake: When Anna is talking to her friends at school, her shirt has a patch of white fabric with the guitar design on it. When she is arguing with her mum about her door, the white patch of fabric is gone. It changes back and forth several times.
Continuity mistake: When Pei Pei calls for her mother, she comes from the kitchen. She sees Anna and Tess and tries to escape by going back through the door she came from, but when the camera cuts back to her, she is about to enter the other door. (00:49:48)
Continuity mistake: At the wedding rehearsal, when they have just returned to their own bodies, Anna picks up a glass of water to toast but once she raises it, they feature a shot from behind her in which the glass and the liquid in it has completely changed. (01:26:36)
Continuity mistake: While in class, when a student responds about Hamlet, Anna has her right arm raised. A shot later it's lowered.
Answer: You need to think back to the poem... "A journey soon begins, its prize reflected in another's eyes. When what you see is what you lack, then selfless love will change you back." It says a journey SOON begins, not immediately, and it says "WHEN what you see...change you back". Basically the switch happens SOON, the switch back happens IMMEDIATELY.