ALSO AFFECTS 1.7 JUST DIDN'T LET ME PUT IT.
This graphical bug causes the cracks on the side of a block when you try to break it to glow when mipmapping is enabled. It only happens when mipmapping is enabled, and didn't seem to be affected by other graphics settings.

It also requires you to be looking at the block on a tight angle, almost so you can't see the side, but it's definitely noticeable.

One thing I noticed is that it's less bright on 1.7, and only happens when antiostropic filtering is off. On 1.8 and 1.9 snapshot it's much brighter. It's not effected by duel wielding on 1.9 either.

So here are some screenshots, and while I show it only on dirt, it happens on other blocks too such as wood and others.