High MCAT, low GPA often means someone who can rise to the occasion for a high stakes, one-day exam but falters on the day-to-day grind of assignments, quizes and the like over many years. Or it could be someone who was not pre-med at the start and had a lukewarm GPA before deciding on medicine. (in which case, look at upward trend and sGPA). Rarely this is a student or grad of a school with no grade inflation (we have a mental list of these schools - sometimes a committee letter will discuss the distribution of GPA of students at this school or of pre-meds at the school or otherwise put the GPA in context)
Low MCAT, high GPA often means that someone suffers from test anxiety in a high stakes situation or has taken a very undemanding undergraduate major (look at list of courses taken) or has attended a school with rampant grade inflation (we know some of these schools by reputation).