Flashbacks, coming right out of nowhere; you are in the middle of doing something or thinking about nothing serious in particular and suddenly you see a mental image of something totally unrelated.
Happened to me half an hour ago... my brother said something about him not wanting to eat old fries from last week and suddenly I see the porch of a childhood friend's house in my mind. Which I last saw... about 13 years ago.
And I don't see a connection between those to in ANY way.
This happens to me A LOT and it's kinda bugging me.