I'm a teenager from Chicago. I've noticed some particular usages of the words "a hundred" by people around me.
During a running workout, one student was 100 meters from the finish, while another student was encouraging him from the sideline. He said something like:
Finish that last a hundred!
In addition, I have heard the word "hundred" used with two preceding indefinite articles. I can't pinpoint any exact examples from my life, but I think I've heard people say things like this after receiving a grade on an assignment:
I got an a hundred.
I don't believe this way of stating "a hundred" rather than "one hundred" or simply "hundred" is standard in any variety of English I know, and it is certainly not grammatically correct. However, it strikes me more as an interesting colloquial, idiolectal variation than a "mistake."
There are a few ways to analyze the construction. For the first quote, we can initially surmise that "a" is replacing "one," which is a common correspondence for many speakers (but probably not in this context). Although this could be the case for the second quote (a different realization of "a one hundred") and is arguable, I think that second quote reveals a more convincing analysis - namely, that people are treating "a hundred" as one word: "ahundred." I assume that many of the speakers who make this choice do not consciously notice it, and I doubt it would be used in writing.
Does anyone know where the origin of this grammatical construction might be? Does it come from a particular dialect? Is it only common in younger urban Americans, or can it (or anything similar) be heard in other parts of the English-speaking world?