What does “Error in namespaceExport(ns, exports) : undefined exports” mean?

Question

When building a package, I got the error

Error in namespaceExport(ns, exports) : 
  undefined exports: FooBarBaz

What does this mean, and h

Answer 1

I had a very stupid typo: in roxygen2 skeleton, I copied what was meant to go into #' @return field, to @export.

Should have been:

#' @return new data frame
#' @export

Instead, I had:

#' @return
#' @export new data frame