S3method(gen, cls) without a function gen.cls, which fails loading the namespace
a NAMESPACE import from a package not in Imports or Depends ("Namespace dependency missing from DESCRIPTION Imports/Depends entries")
importFrom(pkg, f) of a name pkg does not export, which fails loading the namespace
library(pkg) of the package itself in its own code, which fails when it is not on the library search path
test code R CMD check never runs, or that runs the tests of another package
test or vignette code that does not parse, so running it fails
a vignette without the metadata R needs to find and build it
test or vignette code writing a file into the working directory, which R CMD check notes as left behind
a tests/*.Rout.save without the test it is the expected output of
Rust code without the SystemRequirements CRAN asks for
a package without a NAMESPACE file, which no package may be since R 3.0.0
a DESCRIPTION Type other than Package
an installed package (or a build left inside the sources) handed to the check instead of a source package
export(x)of something the package does not define, which fails the installation ("undefined exports")