When working with UIStackView within Interface Builder I would frequently encounter errors that would tell me the an impossible co-efficient could not be reconciled within auto-layout. I fixed this by not using UIStackView. This was a difficult decision to make and only came after losing a few hours to these issues. I had actually argued to target iOS 9 in large part because of my excitement about this new tool; hopefully Apple will fix it in iOS 10. 😞
The aforementioned error:
Cannot find an outgoing row head for incoming head during optimization of variable with near-zero coefficient, which should never happen.
Yet another, more frustrating problem I was hitting involved the reliable crashing of Xcode when working with 4 horizontal stack views within a larger vertical stack view. I lost half a day to this and once again accepted that I would have to pretend UIStackView does not exist.
Someone let me know if they ever fix it. 😒