When I first saw images from Gurren Lagann, and before I had watched it, I had a totally different idea what the story was about.

I thought it was set in a Zoids-like world, where people competed in giant robot battles. Simon, a poor, robot battling enthusiast dreamed of competing, but was held back by his finances. Kamina, his best friend, helped him acquire a broken-down, second-hand mining robot, and suggested he enter competitions with it. To their surprise, they excelled through battle after battle in the world championships, even overcoming Viral, the rich, former champ, and his expensive robot.

tl;dr

First impressions on series before you actually watched them/read about them.