ZF-9531: Incorrect headers when used with other services.
I was using this after I ran akismet to check for spam and was getting errors. The problem was that headers set by akismet were not reset before recaptcha sent it's request. This can cause unexpected results or errors. What was actually happening in my instances is the Host header was not reset and the recaptcha server wouldn't respond to the incorrect domain.
I noticed this happening in 1.7.4 and it seemed to be a problem up until at least 1.10