Final answer:
The incorrect pronoun shift in the sentence 'The cinnamon was so old that they lost most of their flavor' is the use of the pronouns 'they' and 'their'. The correct sentence should be 'The cinnamon was so old that it lost most of its flavor'.
Step-by-step explanation:
The pronouns "they" and "their" are used incorrectly in the sentence "The cinnamon was so old that they lost most of their flavour." The pronouns don't agree in number, hence this is wrong. "The cinnamon was so old that it lost most of its flavour" is the proper sentence to use instead of this one.