There was some fight with the Hulk (early 70's maybe), where Hulk broke apart mjolnir. Picture is in Hulk Respect thread. I want an explanation on that one, for what was happening the whole issue. Was that the real hammer? Because Hulk has broken his hand on mjolnir many other times.
Other than that, it happened twice in the 1990s; once during Roy Thomas second run ( around 1994), and again during Jurgens/Romita's run at the end of the 90's, in a confrontation with Perrikus.
Climatic issues of the vol. 2 series, Thor brought the hammer to Surtur to be reforged; don't know what happened to it prior.