* People doing photography wrong (shooting too many cats, doing HDR, tilting horizons, centering subjects, using toy cameras etc). The internet appears to contain shiploads of rules and moaning about what should not be done.
* Distinguishing "street photography" from regular photography that just happens to take place in an urban environment
* Avoiding clipped highlights and blocked shadows with religious zeal
* Technical (as opposed to real world) lens testing, MTF graphs
* Everything being already photographed by someone
* Film misloading with M4 and later style bodies. When I got my M4, I downloaded an M6 manual from the web somewhere, read the loading bit, dropped film in camera as described (dead easy) and wandered away to shoot.
Then, months later, I read about leaders coming loose from the spool and how the old style loading is "better" and so on. Oh well. I guess it's not easy then.