I have also encountered some different styles over the years.

A good one that I saw involved three shapes. You had a triangle, a sphere, and a cube. There were three patterns. Striped, polka-dotted, and plain. The shapes also had textures. Some shapes were smooth, others had fur. There were 3 backgrounds. I think one was brick, one was flowy colours, but I forget what the third background was.

Anyways, out of those options, you were generated a random combination of two shapes, 2 colours, a texture, and one background. The captcha generated it's own 3 randomized images, but the fourth image matched your generated image. The placement of the fourth image was also randomized.

I have to be honest, I was tipsy when I used it and it kept me out for longer than I'd like to admit haha.