If you are referring to the metal blob to the northeast of the 6 ...it is the end of the ribbon piece holding Georges ponytail. Search the web for "image of bicentennial quarter" and select one of the images displaying a proof bicentennial quarter. The proof clearly shows what the design entails. Your coin does not show all detail. It could be because the die was polished, which removed some of the detail, or, most likely it is the result of die deterioration. An overused die will mute the details such as what your coin shows. Add the fact that it has 43 years of circulation and its easy to conclude that you have a worn out coin, not an error, and worth only face value. Spark
here's a regular 76 Bicentennial quarter. It has the same "mark" as yours, which is part of the design. If you ever see anything out the "ordinary" just look at nicer examples of your particular coin. I use usacoinbook.com to use for quick comparison. Most of the time anything "different" is usually damage.
Now that you hopefully know what it is with the last previous posts.. What nail did you mean? A humam nail or a nail you hit with a hammer?
I don't think anyone uses that very frequently OR knows about it. I'd have given you a best answer, for sure!