What’s wrong with CAPTCHAs, apart from the accessibility problem they create? This:
Note the snowclone in Erinn’s article title Unreadable captchas considered useless. This is a variant of the “X considered harmful” snowclone — clearly more widespread in the coder community than elsewhere — whose inventor is not Edsger Dijkstra, even though it does go back to his famous article of 1968 entitled Go To Statement Considered Harmful (Communications of the ACM, Vol. 11, No. 3, March 1968, pp. 147-148).
Via Erinn Clark.