I just voted to close What language(s) would animals speak? as 'too broad'. When I again clicked on flag there are suddenly two 'already used' notifications:
Is this a bug?
Sorry for not drawing free hand ellipses
When you don't have the reputation needed to cast actual close votes, what you're actually doing is flagging to suggest closure. Once you've flagged for closure, the other options that suggest reasons why the question should be closed aren't available, as you've already flagged that the question should be closed.
Notice the others are still there, e.g. quality related, spam, other. This is very similar to how you can't cast 2 close votes on a single question.
If you make a mistake in the reason for closure that you picked, just use 'other' and leave a note to the moderator that you actually meant to use another.