Let G $G$ be a group, G'=[G, G]$G'=[G, G]$.
"Note that it is not necessarily true that the commutator subgroup G' $G'$ of G $G$ consists entirely of commutators [x, y] : $[x, y], x; , y \in G G$ (see [107] for some finite group examples)."
Quoted from http://www.math.ucdavis.edu/~kapovich/EPR/ggt.pdf page 8.
Anybody can provide the examples? I can't find the book [107].

