Well, completely random thoughts do occur for me, but what happens even more often is a trail of thoughts that lead to something absurdly off topic. I can't think of any instances specifically because I usually discard them immediately afterwards, but they would go something like this:
Hmm, my speakers don't sound so good, wonder what could be the matter. Well, the coils inside could be getting kinda worn. Hey, I remember learning about the coils back in physics class. Ya know, my physics professor was cool. I think he was researching stuff relating to plasma. Plasma, lightning, hmmm think it might storm tonight. Better go put cover up the lawn mower before it starts raining.
My speakers don't sound so good, so what do I do about them? I go outside and cover up the lawn mower, that's what I do

. Some times when this happens the trail is really friggin long, and I get a kick out of just trying to follow my train of thought back to see where I began (or sometimes I even need to remember what I was thinking about in the first place heh).
-