The blow-up of $C_5$ shows that the bound $n^2$ in Erdős Problem 23 is tight: any bipartite subgraph must omit at least $n^2$ edges.

There exists a triangle-free graph on $25$ vertices such that at least $25$ edges must be removed to make it bipartite. The balanced blow-up of $C_5$ with five parts of size $5$ witnesses this.