LeechCraft

% wget "http://lurkmore.to/index.php?title=%D0%A1%D0%BB%D1%83%D0%B6%D0%B5%D0%B1%D0%BD%D0%B0%D1%8F:NewPages&feed=atom&hideliu=&hidepatrolled=&hidebots=&hideredirs=1&limit=50&namespace=0" 
--2013-08-23 15:56:45--  http://lurkmore.to/index.php?title=%D0%A1%D0%BB%D1%83%D0%B6%D0%B5%D0%B1%D0%BD%D0%B0%D1%8F:NewPages&feed=atom&hideliu=&hidepatrolled=&hidebots=&hideredirs=1&limit=50&namespace=0
Распознаётся lurkmore.to (lurkmore.to)... 85.17.124.181
Подключение к lurkmore.to (lurkmore.to)|85.17.124.181|:80... соединение установлено.
HTTP-запрос отправлен. Ожидание ответа... 404 Not Found
2013-08-23 15:56:45 ОШИБКА 404: Not Found.

Shouldn't it fail?

$ wget "http://lurkmore.to/index.php?title=%D0%A1%D0%BB%D1%83%D0%B6%D0%B5%D0%B1%D0%BD%D0%B0%D1%8F:NewPages&feed=atom&hideliu=&hidepatrolled=&hidebots=&hideredirs=1&limit=50&namespace=0" | head
--2013-08-23 16:38:20--  http://lurkmore.to/index.php?title=%D0%A1%D0%BB%D1%83%D0%B6%D0%B5%D0%B1%D0%BD%D0%B0%D1%8F:NewPages&feed=atom&hideliu=&hidepatrolled=&hidebots=&hideredirs=1&limit=50&namespace=0
Resolving lurkmore.to (lurkmore.to)... 85.17.124.181
Connecting to lurkmore.to (lurkmore.to)|85.17.124.181|:80... connected.
HTTP request sent, awaiting response... 404 Not Found
2013-08-23 16:38:20 ERROR 404: Not Found.

$ curl "http://lurkmore.to/index.php?title=%D0%A1%D0%BB%D1%83%D0%B6%D0%B5%D0%B1%D0%BD%D0%B0%D1%8F:NewPages&feed=atom&hideliu=&hidepatrolled=&hidebots=&hideredirs=1&limit=50&namespace=0" | head
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  1051    0  1051    0     0   2433      0 --:--:-- --:--:-- --:--:--  3243<?xml version="1.0"?>
<?xml-stylesheet type="text/css" href="http://lurkmore.to/skins/common/feed.css?270"?>
<feed xmlns="http://www.w3.org/2005/Atom" xml:lang="ru">
                <id>http://lurkmore.to/index.php?title=%D0%A1%D0%BB%D1%83%D0%B6%D0%B5%D0%B1%D0%BD%D0%B0%D1%8F:NewPages&amp;feed=atom&amp;hideliu=&amp;hidepatrolled=&amp;hidebots=&amp;hideredirs=1&amp;limit=50&amp;namespace=0</id>
                <title>Lurkmore - Новые страницы [ru]</title>
                <link rel="self" type="application/atom+xml" href="http://lurkmore.to/index.php?title=%D0%A1%D0%BB%D1%83%D0%B6%D0%B5%D0%B1%D0%BD%D0%B0%D1%8F:NewPages&amp;feed=atom&amp;hideliu=&amp;hidepatrolled=&amp;hidebots=&amp;hideredirs=1&amp;limit=50&amp;namespace=0"/>
                <link rel="alternate" type="text/html" href="http://lurkmore.to/%D0%A1%D0%BB%D1%83%D0%B6%D0%B5%D0%B1%D0%BD%D0%B0%D1%8F:NewPages"/>
                <updated>2013-08-23T12:38:33Z</updated>
                <subtitle>Материал из Lurkmore</subtitle>
                <generator>MediaWiki 1.16.5</generator>

Probably, WGet and LeechCraft useragent strings are blacklisted by Lurkmore?

That doesn't explain the other lurkmore feed being added correctly.

Moreover, with wget it's 404, but LC gets a 200 reply and an HTML page.

Found the sucker. AddFeed dialog considers the string as a human-readable form, thus it percent-encodes it for the second time, and the feed isn't found.

Why lurkmore returns a 200 OK for a not found page is another question though.