One thing I keep noticing is the use of background noise or clutter in CAPTCHAS. It’s now well known in the OCR (Optical Character recognition) field that background noise can be easily removed by computers. It’s basically useless at hindering spam bots.
It’s so easy that I was able to clean the following CAPTCHA up in only 20 lines of PHP code.
Here’s how…
At a glance, you can see the CAPTCHA’s background noise has a blue tint. 
Looking at the RGB value of the image in Photoshop, I can see that all parts of the background have a blue value higher than 180.
That’s the only piece of information needed to remove the background.
The code simply loops through every pixel of the image and checks the RGB value of it. If the blue (B) value is higher than 180, color it white.
Here’s the final image. The characters can now be easily separated and identified using OCR software.
So you can see why most background noise is basically useless in CAPTCHAS.