There were human factors at play, but a critical pilot operated relief valve was also faulted. I've seen this noted in many reports as well as all over the net and unfortunately also Wikipedia which everyone references.
What I don't find is what exactly was the mechanical fault of this particular valve. As it turns out there were other failures in plants of similar design in the same valve (from different manufacturers).
I've done what scouring I can do to find out. What do you know about it? As valve designers, it seems like this is the kind of 'learning from history' is a good thing. You might want to read some of the Nuclear Regulatory Commission documents published at the time.Ones that you might be interested in are Bulletins 79-05, 79-06 and their supplements, Information Notice 79-016, and NUREG 0578, "TMI-2 Lessons Learned."  These are all available electronically.
You may also want to look at NUREG 0462 "Technical Report on Operating Experience with BWR Pressure Relief Systems",
dated July 1978.  It's mentioned in Generic Letter 79-027.  I couldn't find the NUREG itself in an electronic format. 
However, there is a link on the NRC public website to contact the public document room staff, They may be able to provide you a copy or be able to locate it for you.
Just a final word though -- be careful what conclusions you draw from these 31 year old documents about today's components.

MORE NEWS