The present study explores potential usability gaps when users switch from a familiar to an unfamiliar mobile phone interface. A within-subject experiment was performed in which nine users familiar with Sony-Ericsson T630 and nine familiar with Nokia 7250 performed tasks on both phones. On average, test subjects spent more time on finishing tasks with an unfamiliar phone than with a familiar one. For two of the four tasks, there was a significant difference in completion time between the first-time Nokia users and the first-time Sony-Ericsson users. The tasks of adding a contact to the address book and sending an SMS to a contact in the address book were performed more quickly by new Nokia users than by new Sony-Ericsson users. The subjective difficulty ranking also showed that first-time Nokia users found the new phone easier to use than first-time Sony- Ericsson users did. Hierarchical Task Analysis is used as a potential explanation, and three other theories that relate to these findings are presented: mental models, habit errors, and emotional attachment.