I'd like to counter the narrative that the Mars climate orbiter failure was caused by an engineering error. Yes, there were errors in mixing up units, but the navigators and flight controllers discovered well in advance that the trajectory was off. They wanted mission managers to approve a course correction burn, but were denied.

"Far more was at fault with the Mars Climate Orbiter than a simple mixup in converting metric and British units" https://spectrum.ieee.org/why-the-mars-probe-went-off-course

A better example might be https://en.wikipedia.org/wiki/Hyatt_Regency_walkway_collapse