1. Googlebot and many others support query part rules; Droids currently supports only URI.getPath() (without query part)
2. %2F represents "/" (slash) character inside a path; it shouldn't be decoded before applying rule
3. Double decoding is used by NoRobotClient.isUrlAllowed(URI uri) (method body; baseURI.getPath(); returns decoded string; then we call another URLDecoder.decode(path, US_ASCII);
4. URLDecoder.decode(path, US_ASCII); - UTF-8 must be used!
5. The longest matching directive path (not including wildcard expansion) should be the one applied to any page URL
6. Wildcard characters should be recognized
8. Crawl rate
9. BOM sequence is not removed before processing robots.txt (http://unicode.org/faq/utf_bom.html, bytes: 0xEF 0xBB 0xBF
and most probably many more defects (Nutch & BIXO haven't done it in-full yet). I am working on it right now...
Referenced (even by Google!) http://www.robotstxt.org/wc/norobots-rfc.html seems at least outdated...
Proper reference: http://www.robotstxt.org/norobots-rfc.txt (1996).
We need WIKI page explaining all rules implemented by Droids; hopefully it will become unofficial standard.
Update from Google: