Re: font-lock is broken

[Richard M. Stallman]

    As you can see, this function sometimes returns non-nil (i.e. it says it's
    found a match) but with no begin/end of match 0.

Is that incorrect?  If so, why did it appear to work ok before?

Maybe the criterion for a bad match has to be written differently
to accord with the practical rules for code that used to work.

    I'm tempted to just revert my patch, since it causes constant run-time
    checks and trips up some pre-existing code, only in the hope of occasionally
    working around some bugs that would need to be fixed anyway.

No, don't do that!  Let's see if adapting a little is better.
Those bugs are rather painful.