Help me solve my cranking issue, what is this noise?

I suppose everyone spends days trying to figure out a simple problem at one point.

In some free time today I started checking grounds/connections with no real luck. I had the battery cables on to where they were loose enough to be wiggled off, but still have good tension on the post.

While bashing my brain over this problem, I bumped the pos cable and noticed a spark. Yanked the cable and somehow there was a black substance on the terminal. No clue where it could've come from. A quick sanding to that terminal to remove the substance and it fired right up.

Battery isn't dead, the pos. cable was connecting well enough. Go figure.