This one started with American Idol. Someone on that show kept saying, “That’s a whole nother thing!” I think it was Randy Jackson. I had heard it before many times, but it really didn’t irk me until that moment. It was that season with the guy with the hair, and there was a lot of time to tune out the singing and instead ponder grammatical errors. And pitchiness.
The thing is, when used in this way, the word “another” is really just like saying “an other,” so when you put an additional word in between them, the “an” is likely to become “a” instead. So Randy should have said, “That’s a whole other thing!” The “n” is lost to the consonant. Maybe I should get in touch with Randy’s people.