What does David Deustch mean by errors are everywhere?
If I sit and come up with a theory to explain something, it will have errors. The parts of the theory that are wrong. The only known way to rid the theory of errors is to come into contact with reality. In my field (software and technology) the way to do that is by implementing the software/algorithm.
Errors are part of every form of knowledge from religion and philosophy to economics. Discovering errors might sometimes take years especially if the knowledge is kept secret. This also means we don’t have to judge people for errors in their knowledge- they didn’t know any better.