Other mistake: Lavagirl burns everything she touches, but in some shots she touches Max's shoulders and doesn't burn him.
Continuity mistake: When Sharkboy and Lavagirl burst into the classroom looking for Max, there is writing on the blackboard ("Science, Chapter Two") written in yellow chalk. Strangely, later in the scene the writing is now written in white chalk, and the handwriting of the text is noticeably different. Then, later it goes back to yellow! (00:16:45 - 00:17:40)
Lavagirl: Row row row your banana split boat, gently down the steam of consciousness, merrily merrily merrily, life is Max's dream.
More questions & answers from The Adventures of Sharkboy and Lavagirl in 3-D
Answer: Lava Girl is always hot to the touch. She can control when she throws magma, but the only way she can have any control over her actual body temperature is to control her emotions, which would naturally be difficult for someone her age. Even then, at her coolest possible temperature, her body temperature would still be very high. There are continuity issues with her body temperature throughout the film, ie, being able to sit on ice for extended periods of time without melting it, but the idea is she always has a very high body temperature.