[XelNika]

That much is obvious. The blog post has an example of something that appears to be non-breaking (at least it would be in many other languages), but actually does break things. Both the library creator and user messed up in the example given, but if you're not intimately familiar with the language, both mistakes seem reasonable.

[kleiba]

The keyword here is appear, but in reality it wasn't a non-breaking change.

[XelNika]

But if you write defensively, like the post suggests, you won't be surprised by bad library writers.

[kleiba]

That is good advice, but my original stance holds: it is not a specificity of callbacks per se that is the problem here.