It looks as if two planchets were mashed together before going to the press, and this it the outcome after being minted. Interesting. -O) Would have been nice to see the front of the coin, and to the make a better judgment.
The fact that the line is exactly the same arc as another cent, along with the fact that a die crack wouldn't follow such an arc, and would go from point to point in the die, would argue that it was caused by another cent.
Possibly an incomplete clip. (Punch cut part way through the strip and stopped. Then the strip advanced slightly and the blank was punched out showing the previous partial punching.)
It's not an incomplete clip, that's for certain. Is the curved line actually incuse? If not, then it's probably just a stain.
definitely not a stain.. looks more like a scratch. half looks to be under the letters and the other half on top. i know this isn't exactly possible so i will take another closer look and hopefully post a half decent pic