I have the following question:

How likely it is that an author carefully read through a paper cited by him?

Not everyone reads through everything that they have cited. Sometimes, if one wants to use a theorem that is not in a standard textbook, one typically finds another paper which cites the desired result and copies that citation thereby passing the responsibility of ensuring correctness to someone else. This saves a lot of time, but seems to propagate inaccurate citations and poor understanding of the work being cited.

The question is thus about what should authors' citing policy be, and to what extent authors should verify results they are citing rather than using them as black boxes.