what happen when coke/soda is mixed with milk

Milk contains casein.  Casein is a protein that, in milk, assembles to form microscopic spheres that are evenly distributed throughout milk.  Milk has a pH of 6.6, which is just below neutral making it slightly acidic. At a pH of 6.6, the casein protein molecules are negatively charged. The negative charge is important for keeping the casein molecules compacted into little soluble spheres. If the negative charge is neutralized, then the casein proteins will no longer for little soluble spheres and they will unfold, become tangled, and precipitate out.  The charge on casein changes with pH.  As the pH decreases, the charge will switch from negative to neutral and then to positive.  Thus, as the pH decreases, the casein proteins will no longer form little soluble spheres, and they will precipitate out.

Coke is primarily a solution of phosphoric acid with a pH of 2.8, which is very acidic.  Adding milk to coke results in the casein proteins precipitating out as discussed above. 

When I saw this question, I expected that the proteins would curdle.  I was surprised that most of the caramel color also precipitated out. The reason the caramel color precipitates is because the precipitated protein matrix has favorable interactions with the caramel coloring.  This means there is a competition between water and the protein matrix to bind the caramel color.  The protein matrix wins, and the caramel color falls out of solution with the precipitated casein molecules instead of staying dissolved in water.

With respect to the implications, it's a good lesson on milk curdling as a function of pH. Be mindful of the pH of a solution your adding milk to.  Don't mix milk and vinegar, lemon juice, tomato juice, etc. unless your planning on making cheese curds.

The temptation to combine your drinks and create strange concoctions is strong in anyone with a bit of natural curiosity, but there are some drinks you just shouldn't mix.
Namely, milk and coke.
Both harmless and enjoyable beverages on their own, they combine to make a strange, unappealing substance that we wouldn't recommend consuming.
The acidity levels in coke causes the milk to curdle on contact, and after the initial contact things start to get weird.
After 15 minutes brown foamy clumps begin to form at the bottom of the bottle. This is due to the phosphoric acid in the coke reacting with the milk; the phosphoric acid molecules attach to the milk molecules making them more dense.
Leave it for an hour and the molecules that haven't reacted with the phosphoric acid will float to the top creating the clear like fluid, leaving a solid, clumpy matter settled at the bottom.
So, what does it taste like?
Whilst we're not brave enough to attempt to weird mix, Youtuber Kill'em FTW was and apparently it isn't that bad. In a video where he drinks the mix he said: "it tastes just like coke, it just looks absolutely disgusting."
Now that we know it doesn't taste completely awful, would you give it a go?