For me, it's when you talk to someone who has something significant in common with you, but proceeds to lecture you about it as though you know nothing about it.
I met a woman at toddler group the other day with two children the same age and sex as my own two children who proceeded to lecture me all about what having two children of that age and sex is like as if this were all brand new information to me.
She knew I had two children the same age and sex as hers - I had pointed them out to her when she asked.
This sort of thing gets on my wick - don't people have the basic conversation skills to do anything other than talk about themselves these days?