Not designed, not intended. / Not designed, not drawn or planned before being made.

Not designated.

(now rare) Having no artful, ulterior, or fraudulent purpose; sincere; artless; ingenuous.

That cannot be designed.

Third-person singular simple present indicative form of undesign

To reverse or fail to implement a design; to design something in a way that deliberately rejects the conventions of designing.

The state or quality of being undesigned.

In a way that was not designed; as an unexpected consequence; unintentionally.