By the 1990s, it was obvious the
NGC had a a ton of errors--nonexistent objects, duplicate sightings, position errors, etc.
An attempt was made to clean it up, but it still had around 700+ missing objects.
An organization known as the
NGC/
IC project got started, with people observing each one, pinning down the correct coordinates, and looking at original 18th and 19th century notes to identify errors.
The number of unidentifiable objects reduced significantly. A few errors were identified that were not corrected, like NGC90 and 91, which were reversed. Too many studies and pictures had taken place, so reversing the numbers would make a hundred years of study
become problematic.
You can find the latest corrections here:
http://www.klima-luft.de/steinicke/index_e.htm
The point is, the
NGC catalog has a lot of errors in it that have not been cleaned up and probably will never be, due to the poor notes left by the discoverers.
Still, having a more corrected version is a good thing, even if some
NGC objects will never be identified (if they were real).
