To the surprise of the engineers, there was a height difference of 54 cm

From Measurement Matters by Eiko I. Fried and Jessica K. Flake

After a long and cold journey of 286 days, the Mars Climate Orbiter reached its destination on 23 September 1999. Rather than beginning its mission, however, the satellite disintegrated upon entering the atmosphere because one software module made calculations in US customary units and fed them into a second module that assumed metric units. Four years later, two halves of a large bridge being constructed across the Rhine came together to connect Germany and Switzerland. To the surprise of the engineers, there was a height difference of 54 cm (21 in) between the two sides: Different measurements of sea level had been used (the North Sea vs. the Mediterranean Sea).