There's a certain type of Recaptcha challenge (the 4x4 single-photo one) that I have a 100% failure rate on. If Google decide to serve me 15 of those in a row, I will invariably fail 15 times until they decide to serve one of the other types for round 16. I'm very close to believing that that challenge is actually bugged and unsolvable for everyone, and simply nobody ever bothered to fix it.