As the Admiral Cloudberg article mentioned above notes, it was actually a metric (liters) to metric (kilograms) conversion error. (Plus fuel gauges that failed in an unsafe mode, and flying with clearly broken fuel gauge.)

Well, it's important to explain that the issue wasn't bad math, it was bad units.

The math was correct, they just did the math for pounds of fuel on board instead of kilograms of fuel on board.

Well that combined with lack of working fuel gauges, had those been working the error would have been caught before they even pulled back.

The sad part is that this isn't the last time.

https://www.simscale.com/blog/nasa-mars-climate-orbiter-metr...

The sad part of that is people continuing to learn the wrong lesson. The real issue with the orbiter was not the specific unit systems, it was assuming a contractor followed specifications perfectly, not testing properly before launching it into space where it can't be fixed, and then ignoring the people who noticed the issue until it's too late to try anything because they didn't fill out the correct paperwork. Using MKS and CGS together could cause the exact same issue entirely in metric.