You go into a hall cupboard for the milk for the coffee you're making and are annoyed to find it isn't there, and only when you walk back into the kitchen and look around blankly wondering why it isn't that you then realise that you were looking in the hall cupboard, not in the refrigerator like you should have been....
Or my personal favourite... arguing with my wife for 20 minutes over a point that she was making only to realise that I was basically making the same point in my head..... so I ultimately came to the same point in the end, which just annoyed her even further... all I can say is thank god for chocolate, it heals all wounds... o.O