In grad school, I lived on the top floor of a small building housing a late-night take-out restaurant (Hoagie Haven, in Princeton, NJ, see also this purity test). The restaurant was on the ground floor and quite likely undocumented migrant workers, employed by the restaurant, had the floor in between. The worst downstairs neighbors I ever had were apparently Egyptian. They smoked ferociously (which came up through the floor and was so bad at one point that I couldn't sleep) and listened to quite loud Egyptian pop music. They were later replaced by workers of some Indian (or regions nearby) descent. The infernal smoke was replaced by pungent curries, which were certainly easier on the nose. The new regional pop music was as unbearable as the original tenants.

Also, needless to say, roaches were a constant issue. It didn't really matter how many roach traps I put out, as the roaches were, no doubt, coming up from below.

If you read the first article I linked above, you'll see them talking about my former apartment. George and Emily were my landlords, and they'd previously lived there themselves. I learned of the apartment vacancy one sunny day while ordering my lunch. "You need a place? We've got one right upstairs." Ahh, the memories.

These days, I'm quite happy to have even a couple feet of air between my house and my neighbors. So far, no complaints. I'd rather have neighbors I can see than live far out in the country, where you might end up with pig farmers or something else generating a stench you can't so easily ignore.