Dreams.

I dreamt that a friend was on an expedition in some parallel universe and returned ahead of schedule (the others are still continuing the expedition). Of course, there was public interest, all sorts of interviews on television, and then I met him, and we talked. Casually, I asked:
-- Listen, are you sure you returned to your universe? What if something got mixed up somewhere?
-- How can we check that?
Well, I started pointing out all sorts of little things to him, like the inscriptions on the asphalt (some strange gibberish and Soviet sickles and hammers for some reason) and so on. Well, everything seemed more or less normal, but it's hard to tell for sure...
A little later, it "turned out" that in this universe, there was a strange tradition of stealing. Not just stealing, but simply... literally taking everything that wasn't nailed down. And even what was nailed down should be taken. Well, in general, we also synchronized with my friend on this parameter; I swiped something from him, and then he swiped my wallet from my back pocket... I was even indignant, like, how could you, the whole wallet? Have some decency!
But if the universes had been mixed up, imagine what an awkward situation it would be...