jdMorgan - 2:29 pm on May 9, 2010 (gmt 0)
That was likely me with the advice about keeping good pitch between your planks...
While search engines' response-handling may indeed be sophisticated and flexible, there is a "higher authority," and that is the HTTP/1.1 protocol documentation.
When working on error-handling (or even out of curiosity), just fire up a server-headers checker and test your server's responses under various conditions. Requests for resources linked within your own site should directly return a 200-OK and content, a 304-Not-Modified, or a 206-Partial Content response. Requests for bogus URLs should return 404-Not Found. Requests for obsolete URLs should return 410-Gone. Requests from malicious clients should get a 403-Forbidden. All of these should occur directly, with no intervening 301/302/303 redirect responses.
Requests for salvageable URLs due to minor link typos, type-in errors, or trailing punctuation added by mis-coded forum/blog auto-linking can be 301-redirected to a corrected URL. But maliciously-malformed URLs should be rejected with a 404.
The method you choose to handle the 'grey area' between these depends on how sophisticated your error-handling can be: If you can use a database to look up 'bad requested URLs' and make a reasonable decision on whether they can and should be salvaged based on the actual words in the query string, then do so.
Otherwise you may want to assess just how "malicious" the bad URL requests you typically receive seem to be and how many bad requests you seem to be getting, and use that to decide whether to 301-redirect them to the correct URLs or to just 404 them on the spot.
On the one hand, if your site is being bombarded with hundreds of ppp-keyword-laden-URL requests per hour due to malicious link-building, then a 404 is indicated. On the other, if you get only a dozen of those a month, but several hundred typo-induced incorrect-URL requests, you may want to just 301 them to the corrected URL. The 404 method is "safer" with respect to fending off link-exploits, but the 301 method is much 'friendlier' to typo-prone visitors and those coming from 'imperfect' links posted on other sites.
No two Web sites are the same, and really, only you can decide what's appropriate for your own. But the HTTP protocol signaling should be done directly, unambiguously, and consistently by returning correct server response codes.