A tad bit. It's just a little pet peeve. A teeny, tiny one. No one needs to take offense, because as far as I know none of you say this.
It is a tad or a bit. Not both. Both are nouns. Using both is redundant.
How do these things spread and become so ubiquitous? I've heard people use this from time to time and had hoped it was a local tic, but today I heard it on TV.