Someone in my department once wrote a string class. An interface like CString, but without the Windows dependence.
One "optimization" they did was to not allocate any more memory than necessary. Apparently not realizing that the reason classes like std::string do allocate excess memory is so that a sequence of += operations can run in O(n) time.
Instead, every single += call forced a reallocation, which turned repeated appends into an O(n²) Schlemiel the Painter's algorithm.
