I can’t think of a time I pretended to know something. There have been times when I thought I knew something and turned out wrong. There have also been times when I pretend I have heard someone or understood them, when I didn’t. I have a girlfriend who is ESL, and her English is very difficult to understand (and I usually am very good with accents) especially over the phone, sometimes I just aha, and nod, or smile, etc.