The kinds of examples I had in mind with that phrase: 1) a bunch studies have been published which each provide some support for claim X, from a variety of different angles, but they were almost all conducted by the same group of 4 researchers. 2) You don't know much about nutrition and then read a book by Gary Taubes; now you have a lot of arguments in favor of low carb diets.

The general pattern here is that the object-level evidence (e.g., the findings of each particular study, or the content of each particular Taubes argument) does not entirely screen off the source. There are various pieces of information which you could potentially learn about the 4 researchers or about Taubes which would weaken your confidence in the whole set of arguments.