If I met John, I would call my mom.
I know this sentence is a second conditional case. It relates to a hypothetical situation.
I don't know whether I could also understand it in this way: The context is, I am not sure whether I met him sometime last summer. But if I did meet him at that time, I would call my mom and tell her about him.
And if one day I find out I indeed met John, I will be sure that I called my mom immediatly after that encounter. This is just my habit. I might have met him, but I am not quite sure about that. My memory can't last that long. The only thing I am sure about is that whenever I meet him, I will call my mom immediately. This is the context.