I once implemented a registration page that asked for year of high-school graduation (it was relevant to the site, and wasn't mandatory). After a couple of day

I once implemented a registration page that asked for year of high-school graduation (it was relevant to the site, and wasn't mandatory). After a couple of days, someone noticed that there were a bunch of registrations with something other than a year in that field (usually the name again), and I was told to check out if the form was messing up. It turned out that my form worked, but those were all spam-bots. It turned out to be a fairly effective way to filter for spammers.