"I'm trying to give up chocolate. Last weekend I saw a delicious cake and I found myself telling myself the only reason I wanted it was to boost my energy levels, hahaha you know the feeling, right?" If they didn't immediately chime in with examples I'd prompt them with "and you know, its not just food, I rationalise all the time" and ask them if they do as well.
Over than half of them immediately came up with their own diet-related rationalisations. Of the other 4 I had the "calling my mum" one above, a couple of people who said they often caught themselves coming up with reasons for why they weren't doing their work, and one "the dog wouldn't like to be taken for a walk in this cold weather".
The reason I mentioned guilt is that a few of them (I didn't count) explicitly used the word "guilty" (like, I'm too tired to work, so I don't have to feel guilty that I'm out drinking) and one person talked about trying to make himself feel better.
And, just to check, did you make sure that all the diet-related examples you got were examples of making false excuses to oneself, and not just examples of e.g. previously intending to diet, but then changing one's mind when one saw the chocolate cake?
Anna Salamon and I are confused. Both of us notice ourselves rationalizing on pretty much a daily basis and have to apply techniques like the Litany of Tarski pretty regularly. But in several of our test sessions for teaching rationality, a handful of people report never rationalizing and seem to have little clue what Tarski is for. They don't relate to any examples we give, whether fictitious or actual personal examples from our lives. Some of these people show signs of being rather high-level rationalists overall, although some don't.