A more thorough explanation:

but cf.

But cf. is a signal used to indicate that a cited source supports something similar to the opposite of the claim just made by the author. This means that the cited source probably conflicts with the author's claim. It is the weakest of the major negative signals. But cf. is the opposite of cf. which is used to indicate a source that supports a claim different from the original claim but analogous to it. But cf. is used before a source that supports a claim that is different from the contrary claim but analogous to that contrary claim.

Example 1: The author claims that all dogs are friendly, but cf. a study by the American Kennel Club which found that some dog breeds are more aggressive than others.

Example 2: The article argues that coffee is bad for your health, but cf. a recent study that suggests moderate coffee consumption can have health benefits.

The examples illustrate how but cf. is used to introduce a source that contradicts the author's claim. In example 1, the author claims that all dogs are friendly, but the American Kennel Club study suggests that some dog breeds can be aggressive. In example 2, the article argues that coffee is bad for your health, but the recent study suggests that moderate coffee consumption can have health benefits. In both cases, but cf. is used to signal that the cited source conflicts with the author's claim.