Adding Brion since he probably knows the answer and if this is a bug or not.

Zero-width space is required for some scripts, to insert a break between letters that would otherwise form ligatures.

Tab ... SHOULD be stripped, lemme check. :)

\t is just an outright forbidden char in titles.

(In reply to comment #2)

Zero-width space is required for some scripts, to insert a break between
letters that would otherwise form ligatures.

Maybe I misunderstand the purpose of these Unicode characters; I'm not a Unicode specialist. I thought that was the purpose of the zero-width non-joiner (U+200C)? Granted, I think the zero-width space (U+200B) also would need to have the same effect as the ZWNJ as it indicates an (invisible) word boundary, but I'd say that's just a side effect. Also, this normally invisible word boundary may be expanded into visible whitespace by text justification according to [[en:zero-width space]]. So right, stripping it would not be right, but maybe it should be treated as an underscore.

Anyway, thanks for the answer, I see the rationale now. Whether it's 100% correct is less important to me. And perhaps people are using U+200B where they should actually use U+200C, and it's thus more user-friendly to treat it that way. I was just trying to understand what the thoughts behind this were.

Tab ... SHOULD be stripped, lemme check. :)

"Outright forbidden": do I see this right that this is rejected at line 2834
http://svn.wikimedia.org/viewvc/mediawiki/trunk/phase3/includes/Title.php?revision=104051&view=markup#l2834
and depends on the configuration of $wgLegalTitleChars?

So, is an installation allowed to define that \t was a legal title character, and if so, what happens then? (Or what would make most sense then?) Replace by underscore?

I see a question and discussion on behavior here, but not sure if there is a valid bug in this report...