C# Associativity math: (a + b) + c != a + (b + c)

Recently I was going through an old blog post by Eric Lippert in which, while writing about associativity he mentions that in C#, (a + b) + c is not equivalent to a + (b + c) for certain values of a, b, c. I am not able to figure out for what arithmetic values might that hold true and why.

via Chebli Mohamed