What were the biggest mistakes that the Emperor of man made?
It should be noted that things going haywire due to them being temporary measures that kept on going after he was indisposed should be "used" with a heap of salt, the appropriation of one of the most effective vehicles the IoM has for SM only being an example of something that isn't his fault (LAnd Raiders).
The Obvious choice is of course, ignoring the warnings about his favourite son falling to chaos and rebbeling against him, although sending The Wolf after Magnus was almost as bad. (Hmm, let's see who hates Magnus the most and will go insanely overboard when bringing him in...The Psychotic magic hating Wolfman, of course!).
Trying to infiltrate the Webway is a more questionable mistake, since the gains were truly immense in potential, and it went to hell due to Magnus's sorceries piercing all of the psychic shielding.
Ignoring the advice of the Eldar might be reasonable due to the fact that they're a scheming, manipulative bunch of monkey hating bastards, and he had much better precog than them. (Most of the time).
Not telling anyone, not even his sons about the Warp is VERY, VERY questionable. On one hand, it prevents knowledge or corruption, or spreading of it, on the other hand, telling it to simply the highest eschelon of the Imperium (As in, those with rights of access to him personally) would have prevented acts such as the ambush that took Horus down, or might have kept Abaddon & Co from shoving Horus into the Serpent's lodge.
Not setting himself up as a belief fuelled Warp-god I'll forgive
