UE 4.15.1
Bug causes project to crash on every startup attempt (before editor opens).
Cause: Creating a UIWidget blueprint with very low disk space (same drive as engine content).
The fix was to delete the offending blueprint that was created. I can see this becoming an issue for source control, if a user inadvertently uploads an erroneous asset file on a Friday. Maybe a startup check for minimum hard disk space would help mitigate.