When viewing a question, when I hover over the star icon (under the voting module) the tooltip says I've made it a favorite when I haven't yet.
Do you mean it says "This is a favorite question (click again to undo)"? The tooltip is telling you what will happen if you click. Just like if you haven't down-voted a question yet, the tooltip tells you you already think the question is not useful.
In other words, the tooltip is telling you what performing the action subsequently means, not what you may have done with that entity in the past.
That text is a little ambiguous with its text:
since the phrase "click again to undo" seems to say that you have already done something. When you hover over an answer upvote it hints:
and then after it is clicked it says:
similarly for the question upvote, question downvote and answer downvote tooltips.
We should probably reword the favorite question tooltip to something like:
and after it is clicked: