First the spacecraft was asymmetric, causing some issues with the stabilizing flywheels and the onboard thrusters (used for major course corrections). Second, the person doing the calculations for the major course corrections noticed that the burn time (calculated using the bad units) didn't look right compared with previous missions. However, his management made him prove that the calculations were wrong, instead of proving they were right (presumably knowing that they would be different, given the first point about the asymmetries). He didn't catch the units error, and since he couldn't prove they were wrong they went along as if nothing happened. The article was really pointing out that while this was a technical error, the more fundamental issue was a management and culture issue. To me this made for an interesting case study in how to handle unknowns in a mission critical system - assuming things are wrong until proven otherwise, not vice versa.
(I don't seem to have the Spectrum issue with me, but I seem to remember it had some other articles about related management/culture failures).