Hacker School has a set of "social rules [...] designed to curtail specific behavior we've found to be destructive to a supportive, productive, and fun learning environment." One of them is "no feigning surprise":

The first rule means you shouldn't act surprised when people say they don't know something. This applies to both technical things ("What?! I can't believe you don't know what the stack is!") and non-technical things ("You don't know who RMS is?!"). Feigning surprise has absolutely no social or educational benefit: When people feign surprise, it's usually to make them feel better about themselves and others feel worse. And even when that's not the intention, it's almost always the effect. As you've probably already guessed, this rule is tightly coupled to our belief in the importance of people feeling comfortable saying "I don't know" and "I don't understand."

I think this is a good rule and when I find out someone doesn't know something that I think they "should" already know, I instead try to react as in xkcd 1053 (or by chalking it up to a momentary maladaptive brain activity change on their part, or by admitting that it's probably not that important that they know this thing). But I think "feigning surprise" is a bad name, because when I'm in this situation, I'm never pretending to be surprised in order to demonstrate how smart I am, I am always genuinely surprised. (Surprise means my model of the world is about to get better. Yay!)