Continuity mistake: When Tess and Anna (in each other's bodies) go to the Chinese restaurant to get switched back, Pei Pei says, "No, you get back here" to her mother and she has a serious look on her face. When the shot changes, she is suddenly smiling very widely at Tess and Anna.
Continuity mistake: When Anna (Tess) wakes up and realizes she is in her daughter's body, Harry comes in and says that mom is dead. She goes into her room and sends Harry down stairs and she says he could eat her (Anna's) cereal. Harry goes down stairs and when Anna (Tess) is going to try to wake her up she reaches with two fingers to poke her and in the next shot she only poked her with one finger.
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: 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.