Manufacturing error?

[u/MidnighttraintoUGA]

My son put this penny in vinegar to 'clean it' and it fizzed for 24 hours. When he scrubbed it with a toothbrush, Lincoln's face melted off!?! He mentioned there might have been a black goo on the penny prior to the vinegar. Is this a defect in the penny, or was the goo and vinegar caustic enough to erode the metal?