An author can intentionally produce the marks 'the cat is on the mat' with the intention of using those words in English, while also intending the words to mean that dogs are carnivorous.

Even if we grant for the sake of argument the not-at-all-obvious premise that marks without intentions are meaningless, the proposition that a text means whatever the author intends does not follow (see Sinnott-Armstrong 2005; Berman 2009, 47-49) – and it is false. Given the premise, the marks “the cat is on the mat” would have no meaning unless their maker produced them intentionally. So, let’s assume that an author produces those marks intentionally – specifically, with the intention of using