There are multiple causes of electrocardiographic ST-segment elevation which are well-known to mimic STEMI and often are a cause of misdiagnosis of STEMI. These are:
Whenever there is doubt regarding whether you are dealing with a STEMI or a mimic, look for reciprocal ST-depression. Most of these will not produce ST-depression (LVH, LBBB, Pacers, and hyperkalemia WILL). The other key intervention is to perform serial ECGs and look for evolving changes, which strongly points to the presence of a true STEMI.