Not making eye contact is probably a form of self denial. Making mistakes sucks because you feel like a dumb ass for a while. So she doesn't look and tell tells herself anything she has to in order to not have to deal with those feelings. Unforutantely I'm convinved that sometimes feeling like a dumb ass is an important part of making less mistakes in the future.

Of course by yelling you probably help force her into a self defensive state of mind. Not only would she have to be willing to be honest about her mistake, but now she has to do it while someone is attacking her.

There are certainly more effective ways to respond. A shrug that says, "Aw man I'm a little bit dissapointed," can put even more pressure on her to admit to her self that she screwed up. At the same time it makes it harder for her to simply blame the whole thing on a guy who seems like a nice, forgiving fellow.