I took a new bit of camera kit to Scotland - a variable neutral density filter. I had visions of lovely arty long exposure photos of various waterfalls, which didn't really happen!
The filter works by being two polarisers, so things darken as you rotate one around the other. Unfortunately this also means you can get some nice interference patterns. I suspect this happens even if you spend loads of money on a proper one, rather than £7 from amazon marketplace...
Here's a bit of the river Etive to demonstrate. 'min' and 'max' settings on the filter, respectively:
Very obvious what the problem is! This wasn't always obvious through the viewfinder, so I ended up with a lot of crap photos with darker patches.
To be perfectly honest, I rarely needed the filter anyway - a tripod would have been far more useful as even with fancy image stabilisation you can only hand-hold for so long before camera shake ruins everything.. And tripods/long exposures are not so appealing when it's pissing it down with rain/hail!