A while back, I noticed that "сосе́д" is "сосе́ди" in the nominative plural and that "чёрт" becomes "че́рти." With "чёрт," my guess is that the "ё" affects the following letters, but with "сосе́ди," I have no idea why it isn't spelled "сосе́ды" like other words that end in a "д" (e.g., под -> поды́, вид -> ви́ды, ряд -> ряды́, наро́д -> наро́ды).
I realize there are many exceptions to the various Russian rules concerning spelling (as well as other types of declension and various aspects of grammatical structure). The ones that are completely different but seem to follow a pattern (e.g., those that take а/я instead of ы/и, those that drop letters, ...) I've been taking in stride, but because these two words here seem to flip the one rule on its head, my curiosity has been piqued a bit more than other exceptions to the rule.
To recap/wrap up, is my speculation about "чёрт" correct or way off mark? Was the word "сосе́д" once spelled differently ("сосе́дь" perhaps?). Or is is it something much more basic that I am just now noticing -- the "д" in "сосе́д" is preceded by a "е" and it, like the "ё" in "чёрт" affects the following letters in a similar way (turning them soft). Then again, that would not explain the plural of "отве́т" (отве́ты), "след" (следы́) and others like them. So, if anyone has any insight into these two words, thanks in advance for sharing your knowledge with us.