Asset packs frequently come with an animated clone of the default UE4 Mannequin and some bespoke animations.
However, those anims rarely work from one version of the Mannequin to the next and retargeting crashes my PC 100% of the time, I’ve attached a log to this from UE4.14, but its happened in previous versions too. link text
Examples here include the default mannequin anims being incompatible with the mannequin that comes with the free extra anims from Epic, and the anims from that pack not working with the default mannequin.
This is also a compatibility problem with other packs out there, even those submitted by the same person.
It feels UE4 could do with a better way of managing the default mannequin skeleton & rig, for projects that use it, to avoid lots of doubling up on the mannequin assets.