Yep. The present parser doesn't throw errors, it
just gives you
something odd. Throwing errors in a new parser would be unacceptable.
"Everything I can't process just gets put through" would be
acceptable, I'd think.
Why is it unacceptable? I would think users would prefer to be told
what's wrong rather than have something unexpected happen. Just
passing through the wikitext unchanged requires the parser to work out
exactly which bit has gone wrong, which is often impossible (for
example, in <tag><tag></tag>, which tag has been left unclosed? Should
it output "<tag>" as the contents of a tag tag, or should it output
"<tag>" followed by an empty tag tag? That's assuming it can find
some
way of working out that it isn't meant to swallow the rest of the
article. A simple error message saying "tag tag not closed", either
inline when displaying the page, or as an error on save, would be much
easier and much clearer.)