Not the main mirror -- the mounting of the secondary mirror. IIRC, it was a screwup between metric and English units of measure...
Obviously, it was fixable -- and fixed by shuttle astronauts.
referencing Metric vs. English units; You might be thinking of a space probe that was lost, due to just that exact problem. The Name of that probe escapes me, but in a critical part of the software, the Units weren’t consistent between metric/English. (I recall it crashed, but not sure if it was on the Moon or Mars.)
The Hubble situation involved the physical placement of a mechanical device they use to monitor if the grinding was proceeding equally across the surface. The test device wasn’t actually at the physical location they thought it was from which they calculated the various optical formulas. Somebody used a shim during installation of that device and that gap measurement wasn’t known to the people who used the device.