Using Illustrator 10, I open an old AI file and see the error message "this file uses fonts or characters which are not available or are in a different format than originally specified", and mentions "Myriad Bold", which is not (visibly) used in the file.
It then gives me the choice of replacing the fonts or cancelling.
But this replacement is temporary, I get the same error every time I open the file.
If I open "Type/Find font" it list the five fonts I do use in the file. Not including the "default" font, Times, which I imagined a missing font would be converted to. How can I find whatever object has this font problem and delete/change it?
How can I find whatever object has this font problem and delete/change it?
That's hopeless, as in fact there may no longer be any such object. The font may have been used to generate an element which was later converted, but based on what tool/ function was used, the reference to the font was not cleaned up. E.g. if the font was the current active font for a specific text field, it may persist even if the text was reformatted later. Likewise, symbols, charts and other stuff could still contain a link to the font. The best solution you can try, is to run the expand apperance on all elements you don't need editable, then copy everything over to a new blank document. This may clean out the internal data structure and resolve the issue.
Missing fonts remap to myriad by default. Did you remove it by any chance? I'm pretty sure that Illustrator needs it in order to function.
If it's not showing up in the "find font" dialog in brackets then as Mylenium said, it may be hidden in a symbol or other nestable element.
Also, make sure that there are no locked or hidden objects, as one of them could be the culprit.