holy!!! that maths one is giving me the jitters, and I'm a mathematically educated engineer (albeit, lapsed)!
captcha is a nice idea - but i've yet to see one that is completely user friendly but still can't be solved by a bot that was hacked out in 5 minutes... and i've been looking long and hard.
still, the worst offenders are the ones that don't even work (like where he doesn't have the image loaded in his examples) - surprisingly, the forums for a major games distributor (who shall remain nameless, but think in terms of
hot vapour 
) presented me with an impossible to read CAPTCHA image, so I clicked for a new image... however, the system didn't pick up that I had requested the new image, so even though I got the answer right, it still failed me because it was testing the first one
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Can you digg it?
It call me "Hello Robot" three times. 
Linear Mode
