Vin+2 ground pins oxidation

Development Tools IDE 1.x
1

I use my arduino quite a bit, and have noticed that the two ground pins beside Vin, and the Vin pin have slowly accumulated oxidation inside their female headers. The arduino seems to work fine, but I'm not sure what is causing this. Typically I use 2 or 3 peripherals depending on the project
, all of which seem to be functioning properly. Does anyone have any idea as to what is the likely cause of this?

2

here is a photo

3

Are you sure that isn't rapidly burned plastic?

4

thermal damage?

5

I think it probably is burned plastic. What I'm not sure about are the kinds of things would make that area, specifically, to overheat. Overuse? too high of a current draw from the devices, etc. I usually use those two grounds, for devices, but never use the Vin pin

thanks for the input!