I can totally understand why those would frustrate non-technical users, but on a site like HN I would expect people to know how reCaptcha works.
That said I think reCaptcha has been getting much harder recently due to the arms race with bots. I now sometimes fail 3-4 times in a row.
Why should you even expect that? Recaptcha was interesting in 2006, but anyone not following the news around that time or around the its acquisition by Google might not have learned this.