Men of Iron
The Men of Iron, or Iron Men, were sentient, sapient robot servants created by humanity during the Dark Age of Technology. Men of Stone and Gold were also involved somehow (see below).
Unfortunately, the Iron Men were capable of learning and self-improvement; before long, they became smart enough to realize that the squishy meatbags were dependent on them. This so insulted them that they rebelled against humanity. They were eventually put down, but the rebellion, along with the other dangers of the galaxy, was enough of a disruption to send humanity into an Age of Strife.
Few records remain of this time in history, so the form that these Men of Iron took is not known, but their rebellion still left a huge impression on the nascent Adeptus Mechanicus; even to this day, they strictly forbid the creation of "abominable intelligences" and shun even the idea of self-improving machines, preferring to use relatively tame servitors and Machine Spirits instead.
Some Men of Iron still survive to this day, locked up in ancient pre-Imperial technology archives. Because tech-priests can't keep their noses out of ancient technology, these archives are often inadvertently activated. The Tanith First and Only also discovered an STC factory that had been corrupted by Chaos to produce Men of Iron, and they destroyed it immediately thereafter.
The STC producing Men of Iron may also shed some light on a deeper reason behind their betrayal. It is entirely possible that either the Men of Iron were corrupted by Chaos or the factories producing them were corrupted, maybe even both. Any evidence of this, beyond the discovery by the Tanith, would likely have been lost during the daemonic invasion that followed during the Age of Strife.
The Journal of Keeper Cripias
The most extensive account of the Men of Iron is a piece of lore from the third edition rulebook, journal entry by one "Keeper Cripias" of the great Library Sanctus on Terra. In it, Men of Gold (the "First Men") and Men of Stone (the "Second Men") are also mentioned, with the Gold Men dying out during the Dark Age of Technology and the Stone Men creating the Iron Men sometime thereafter. It is not clear if either of these Men are supposed to be humanity, or if they are both subsets of humanity. They are also called the "Golden Race" and "Stone Race," but the Men of Iron are also called the "Iron Race," and there is also mention of the "human race," and he writes that, at one time, there was no "Race of Man" at all, "just warring factions" -- so it is possible that he uses "race" in a rather different sense than we normally do.
Bear in mind, the Library Sanctus has been subject to nearly forty-thousand years of revision, deletion, and misfiling (and Cripias himself was declared a heretic as well), not to mention how much of it was passed on orally for absurd amounts of time before finally being written down, so who knows if any of this information is worthwhile -- it certainly hasn't been mentioned anywhere since, though the Horus Heresy novels may shed some light on the matter.