Remove before wearing or washing

On the subway ride home from work yesterday, I was sitting with my new sweater on my lap, listening to music, when something struck me as disturbing...

I noticed a small clear circle on the sweater, about a centimeter radius, with the letter "L" printed in black in the middle. That's right, a sticker was stuck to the the sweater I had been wearing the entire day, announcing to the world that I wear a size Large. And I bought that size specifically to layer over my regular clothes because I work in an ice-box known as a high rise, but that's beside the point. I can't believe no one told me. At work. Nothing. At Kabbalah class. Nothing. And we were talking all about sharing in class. And yet nobody thought to share with me the fact that I had a sticker on my chest?!

And then, I made the second disturbing discovery...

A thick tag sewn into the inside of the sweater that said: REMOVE BEFORE WEARING OR WASHING.

I have to admit, it freaked me out a bit. Since I had been wearing the sweater all day, I began to wonder...

What is inside the mysterious tag?
Why was it sewn on the sweater in the first place?
And why is it so important that I remove it before wearing or washing?

Needless to say, I am not wearing that sweater today.