Well, it’s more an observation than a real problem.
For my current CakePHP project I had to set a captcha field to a user registration form. I decided to be “creative” and I set the captcha image as background of the input field. In fact the graphical representation wasn’t bad at all. Unfortunately when it’s used, the captcha word shows wrong image and of course the result was wrong. To be correct the first attempt was always ok, but the second one (let’s say if the user decided to register again) the captcha was always wrong.
The solution came when I removed the image from the background of the field and call it with an <img> tag. This time everything went just fine. This make me think that it’s a cache problem. I didn’t dig it deeply, because for now it’s working and as you can see it’s looking better.
So, an advise from me – never put your captcha image as background of your field, or call through css. Just use good old <img> tag instead.