I keep having this issue where people will speak to me when I'm not expecting it, and my brain just kind of freezes and I can't think of a good response. This used to happen a lot at work when customers would try to make polite conversation, where unless they asked a question, I couldn't think on the fly.
Recently it's happened a few times, once in a McDonald's where some random dude started lecturing me about how the food never decays (which… duh. I know that. I don't eat at McDonald's to be healthy.) and then started ranting about the alphabet? and it didn't occur to me until afterward that the proper response probably should've been "Leave me alone" instead of awkwardly going along with what he said.
The second time was in the line in a store when the lady in front of me observed what I was holding and commented about having seen it on the telly. I didn't know what to say and didn't end up saying anything, which I could tell made her insecure because she apologized for speaking to me randomly like that. I assured her that it was okay, but I still feel like the whole exchange could've gone better if I had just responded the first time she spoke to me.