[evgpbfhnr]

You don't actually need much, for a form I used to get spam in I just added a "write 42 here" so anyone who actually cares to read would be able to fill it. spam fell to 0.

(for a site with a slightly higher profile this wouldn't be enough, but for a minor corner of the internet with no ill intent actually aimed at it that turned out to be enough to block the fuzzing "fill all the forms" spam)

[electroly]

As contrasting experience, I did that (a simple math problem) on our contact form and it did NOT drop spam to zero; our spammers were too smart for that. Even an actual reCAPTCHA didn't completely eliminate it (although it mostly did, enough that it's fine for us).

[kqr]

Similarly an empty input field that is css'd to be outside the viewport is often filled by spambots but not humans. But I like the edge case UX of your idea more.

[jeffhuys]

Just watch out that Chrome’s autofill doesn’t fill it in. Cost us a huge chunk of new signups until we found out. Chrome ignores autofill directives under some circumstances.

[kqr]

It's also visible for users with CSS overrides and/or other browser inpairments. The more I think about it the more strongly I prefer the "type 42" explicit input field.

[thih9]

You can label it “leave this field empty”, with a placeholder or similar - then it’s the same explicit instruction as “type 42”.