Case 1: Pushing stacked minecarts glitches them through blocks. 

What I expected to happen was...:
I expect that the whole stack of minecarts would get pushed all together and stay in the same exact spot. 

What actually happened was...:
Instead the minecarts would get dispersed and some (in a random amount) would get glitched inside the block that is pushing them.

Steps to Reproduce:
1. First place a line of rails (any) on a flat floor
2. Then place a block on any end of that track
3. Then stack a couple of minecarts (any) on the rail that is directly next to the block you just placed

4. Then place a piston facing the same direction as the track behind the block you placed, you should have this : (Make sure that the piston isn't pushing more than 12 blocks)

5. Now finally, trigger the piston and observe, you realize that some of the carts got glitched inside the blocks like the following:

Note that the above scenario doesn't apply only for stacked minecarts, minecarts placed in succession CLOSE TO EACH OTHER can also lead one to glitch through a block like the following picture : 

Case 2: Multiple Minecarts on an powered rail are activated at different timing.

What I expected to happen was...:
I expect that the whole stack of minecarts would get activated at the same time and move all together as one.

What actually happened was...:
Instead the minecarts would get activated 1 by 1.

Steps to Reproduce:

1. First place a line of powered rail on a flat floor

2. Then place a block on any end of that track

3. Then stack a couple of minecarts (any) on the rail that is directly next to the block you just placed, you should have this:

4. Finally activate the rails, and observe, you should obtain this: