One address was a complete dummy, used to test the integrity of a Saudi-backed group (they proved trustworthy, btw). The dummy account was idle for several years, until a 1-byte message appeared, which showed whoever was probing that the address was real. The amount of spam has been rising fairly steadily since then. And the rate of real messages. That dummy account got email from a woman wanting to talk about vacation pictures (I set her straight), and then nothing but spam until a few months ago, when somebody accidentally set it as the account reset location for his membership in some Belgian porn site. (I gather he got things straightened out, since the reset messages only showed up for a week.)
On another account, for the past year, I've been getting messages inviting some woman I've never heard of (and whose name bears no resemblance whatever to the account name!) to visit one or the other college. There's typically no place to ask to be removed from such lists. On another account, which I use for Craigslist, I just got Verizon's billing information for somebody in New England with a new cell phone. I spent a quarter hour on the phone being a good citizen about that one.
There were two other similar errors that I can't recall the details of. In sum, over the past 3 years, on 5 accounts, I've found 6 errors. Last year Google said there were over a billion accounts. Shall I extrapolate that and say there are of order 400 million errors per year? (plus or minus 160 million :-) ).(*) Not google's errors, of course--typos or misunderstandings or made-up addresses. At Chicago's Museum of Science and Industry, they used to have a demonstration of phone number reverse lookup--type in a number and it would give the address. Pick a random number, and the chances were it was a real phone number...
(*) I know, the look-elsewhere effect--I wouldn't be posting if there hadn't been something recent--the real error rate is lower, of course.