I think I go annoyed of how often do Stackexchange sites like Serverfault.com and StackOverflow.com want to verify I'm not a robot. I'm already coming close to doubt myself - maybe I am?

For example, sometimes when I write a to long question or misspell a tag, I get asked if I'm not a robot first, then I get my error, correct it and press to submit, and ... am asked for captcha again.

What the reasons could be? Maybe I am using a suspicious IP address or something alike? Or maybe there's a way I can make the server less suspective by setting up something in my profile?

I use prepaid O2 Telefónica 3G in Prague, CZ. I run Ubuntu 10.10 and use its standard Firefox 3.6.10 with some extensions like NoScript, BetterPrivacy, etc (but no TOR, proxies or alike).