Why you shouldn’t use cheap UV lens filters

I've read about the occurrence of weird flare appearing in photos when you have a cheap UV filter on your  lens, but this is the first time I'm able to produce it.

First, a list of equipment used:

• Nikon D300 digital SLR camera
• My brother-in-law's Tamron 90mm f/2.8 macro lens
• Cheap, no-name UV filter on the macro lens

So, I'm indoors with my back facing the doorway.

There's a bright source of light, which is actually strong mid-day sunlight reflecting off the walls of a neighboring house.

I attempt to shoot a reflection of myself in the mirror.

Here's what I get.

Looking at the flare a little closer now, it might be caused by the reflection of the bright spot in the mirror on the filter.

I now wonder if the same problem would occur with more expensive filters.

Anyway, I removed the filter, and re-shot the photo, and no matter how hard I tried, I couldn't get any flare in the photo.

None of my lenses have a filter on them, a decision which was influenced by Thom Hogan's article on filters.