The limitation of over-engineering as a safety technique is that the extra weight and volume may begin to contribute to the very problem that it was intended to solve. No-one knows how much of the volume of code of a large system is due to over-engineering, or how much this costs in terms of reliability. In general safety engineering, it is not unknown for catastrophes to be caused by the very measures that are introduced to avoid them.
How Did Software Get So Reliable Without Proof?, C.A.R. Hoare