I used to work for an ad network (BSA, Carbon Ads, etc) that does not intrusively track and believes in the effectiveness of quality one-ad-per-page. Try contacting @toddo on twitter or email me (in profile) and I'll forward it over.
Anyhow, I'd love to see something like this. I've switched to DDG but often have to reach for Google if I can't find relevant results.
The real issue here is that it is not immediately obvious that this is actually an interface - one expects the second `err` to be *FormatError due to the signature - it happens to implement the error interface so it goes unnoticed.
Shadowing in a different scope allows it to work properly as well.
I admit that I've never actually run in to this issue in practice as I generally return custom errors as `error`, but I can see why it would confuse someone.