Malaysia Airlines says the aircraft had been repaired by Boeing and was certified safe to fly. They dismissed the possibility of a technical problem.
JAL123 had been repaired by Boeing, and had been certified safe to fly.
Please note that I am not suggesting the idea that MH370 was lost as a result of post-repair structural failure. I do not believe there is enough information at this point to advance any hypothesis. We need more hard data. There is very little hard data at this time.
I am merely noting that there is historical precedent for an inadequate manufacturer repair leading to an in-flight structural failure, with hull loss, and with massive loss of life. It is one of many different possibilities.