What critical flaw was famously corrected during Hubble's first servicing mission in 1993?

Spherical aberration in its primary mirror

The initial deployment of the Hubble Space Telescope was nearly marred by a fundamental optical defect known as spherical aberration in its main mirror, which caused images to be blurred rather than sharp. This issue was famously rectified during the very first in-orbit Servicing Mission conducted by astronauts in 1993. The solution involved installing corrective optics, essentially eyeglasses for the telescope, which perfectly counteracted the flaw in the primary mirror's shape. This successful repair transformed the mission from a potential failure into one of the most scientifically productive observatory projects in history, highlighting the importance of the in-orbit serviceability designed into the telescope.