As we all know, the HubbleTelescope has a built-in vision problem: The primary mirror has a "spherical aberration" -- its shape isn't quite right.
[From EndToEndPrinciple (violated, in this case)]
"The HubbleTelescope didn't need to be tested end-to-end; the components had been proven correct." ;->
It appears that of the three types of UnitTests that were applied to the main mirror, one showed OK, but the two others indicated a spherical aberration. The passing test was trusted, and the failing tests ignored.
See "Why the Hubble Space Telescope Went Wrong?" at http://www.uoguelph.ca/~ebaig/brian.html
"Scientist says errors discounted in making of Hubble" at http://www.chron.com/content/interactive/space/missions/sts-103/hubble/archive/900916.html
"NASA Fact Sheet," "Hubble Space Telescope's First 125 Days" published by the NASA Marshall Space Flight Center, Huntsville, Alabama, September 1990 -- http://www.jsc.nasa.gov/er/seh/hubble.html
"Overcoming Hubble's Troubles" at http://www.ex.ac.uk/nlo/news/nlonews/1994-01/9401-12.htm