The Hubble Space Telescope's primary mirror was too flat, meaning it was less concave than it should have been. This was a manufacturing error, a flaw that resulted in a blurry image.

Here's why:

* Concave mirrors curve inwards, like the inside of a spoon. They are used in telescopes to focus light.

* Convex mirrors curve outwards, like the back of a spoon. They are used for things like rearview mirrors and can't focus light for imaging.

The Hubble telescope was designed to have a very specific curvature in its primary mirror. This curvature was crucial for focusing light and producing sharp images. However, the mirror was slightly too flat, which meant it couldn't focus the light correctly, leading to blurry images.

This flaw was famously corrected with the installation of corrective optics during a space shuttle mission in 1993.