A series of repeated TIAs characterized by identical neurological deficits is often a warning sign that a major vessel contains an unstable plaque on which thrombi are repeatedly forming (and causing ischemia) and then fragmenting (and allowing blood to flow again). The neurological features of these TIAs are often indicative of the deficit, or part of the deficit, that would be produced by the impending "completed" stroke.
It is less likely that cardiac emboli will repeatedly produce the same deficits--they are able to travel to any part of the brain. In some individuals, TIAs may result from decreased perfusion of large arteries that are already partially occluded by plaque. This might occur with fluctuations in blood pressure produced by overzealous medication or by various cardiac arrhythmias.