iOS 10: NSInvalidLayoutConstraintException: Constraint improperly relates anchors of incompatible types

You perhaps are doing something like this:

NSLayoutConstraint(item: viewA, attribute:
                .leading , relatedBy: .equal, toItem: parentView, attribute: .top, multiplier: 1.0, constant: 20)

So you do not stitch the correct anchors together like .leading and .top. In this case it must be .leading and .leading.

Interestingly this wasn't a problem with iOS9.