what would you call someone who always focuses on the irrelevant bits during an argument...?
someone who, if you said, for instance - "when you went to your mums at lunchtime i asked you to pick up my scarf that you left there, i told you it was very important and you didn't get it" - and their response is (shouting) "i didn't go there at lunch time, i went at about 2pm!" ...
and generally argues about minor facts that don't matter at all, and change nothing in the row.
i don't mean someone who is using deflective techniques, to try to spin the row around onto the other person, or to change the subject, or to stall for thinking time, but someone who is genuinely just being pedantic about 'facts' and gets hung up on them being correct
If they do this unintentionally and (almost) all the time, then I believe this may be a sign of one of the various types of autism - possibly Aspergers. I think I have something similar, where I misunderstand what's being said because I cannot adjust my interpretation to the context, then get into an argument about whether someone failed to express themselves clearly. On the other hand it could just be someone under pressure who hasn't learned how to defend themselves confidently yet.