I can't believe that happens is a common response to women recounting their experiences of harassment. This response doesn't quite qualify as a silencing tactic, nor is it derailment. But when a man responds to a harassment story by performing surprise, he does implicitly recuse himself from responsibility for creating an environment where harassment can't happen. Performing shock or surprise that, for example, a man would walk up to a woman on the street and pull off her headphones so he can harass her, suggests that the speaker is so insulated from women's everyday lives that he can't imagine it happening (in other words, it's an expression of privilege). While perhaps not intended to question the veracity of the "surprising" story, it is still a response that does not validate women's lived experience. It's also a potentially othering response, since it serves to remind anyone in earshot of how oblivious many men are to women's lived realities.
Anecdotally, harassment is much more likely to be aimed at women (and other people perceived as women) who are walking alone, or in groups of only women, than at women who are accompanied by men. This potentially explains much of the reason why some men who don't harass find it surprising how universal harassment is. Other possible reasons are that the surprised man doesn't interact with women very much, or that women who he does interact with don't trust him enough to share these stories.
Surprise functions as an excuse for sexist incidents because it implies there is nothing the speaker could ever have done to discourage other men from harassing. The implicit message of "I can't believe this happens!" is "I am a good guy, who would never do such things or be friends with men who do" -- it places the focus on protecting the speaker's reputation rather than supporting women. In fact, there is little difference between surprise and outright saying "Not all men". A more constructive response is to believe the woman making the report (with or without directly telling her that you believe her), then exert influence on male peers to make it less rewarding to harass or to make misogynistic comments.
- "Feigned Shock and Faux Enlightenment", Ashe Dryden