THE EMH

Dr. Zimmerman's EMH is equipt in all the newest ships in StarFleet. B'ellana Torres has modified the one on Voyager with comprehensive data buffers to expand his memory core, but his modificatable heuristic matrix was originally designed for a total of 1500 hours medical use and were to be deactivated between the emergency's. There wasn't enough storage for the creation of a personality.

What his creator didn't consider was that he would replace an entire medical team on Voyager. Because of that he has overloaded his matrix, that's why he makes mistakes and loses his memory.