A) You don't have your Tefillin and the only pair available for you to borrow is for the opposite hand you generally use (ie, your dominant hand), or
B) You have a cast or splint on your arm or hand that normally wears your Tefillin (ie, your non-dominant hand),
Do you wear the Tefillin incorrectly (in the first case, whether on your dominant hand, or on your non-dominant hand by flipping it around, or on your non-dominant has by wrapping it backwards; in the second case by wearing it on your dominant hand or on top of the cast/splint)?
Do you just not wear the Shel Yad?
Do you not wear Tefillin at all (ie, if the Shel Yad is not Halachically wearable, does it negate the value - is it Me'Akev - of wearing the Shel Rosh)?