If this bug is considered a duplicate, please describe why in a comment.

How I Discovered It:
I had constructed a contraption which fires a spread of arrows. I used it without realizing I had disconnected a redstone wire (more on that later). When I finally discovered the error, I had already run the machine 2-3 times and when it fired, the arrows flew much farther than they had before.

What I expected to happen...:
1. The arrows moved when the explosion went off.
OR
2. The arrows flew the exact same distance.

What actually happened...:
1. The arrows did not move. I tested this with /execute @e[type=Arrow,c=1] ~ ~ ~ /tp @e[c=1] ~ ~ ~.
2. The arrows flew more randomly, but farther.

I have attached the results as labeled png's

The Contraption:
It uses a dispenser which is powered by the old-fashioned powered comparator clock (see image below). Connected to it is a strand of redstone wire, 2 long, connected to a block with a redstone torch on the opposite side, which powers a command block which uses /blockdata to fill the dispenser. The dispenser shoots arrows 7 blocks down, aimed at the center of a 3X3 square of slime blocks.

1 block behind the slime pad and starting 2 blocks above is the tnt holder, a 2X3X3 configuration of obsidian, which is missing 1 obisidian in the center of the 9X9 surface on the side nearest the slime pad.

There is a button labeled "Start" which, when pressed does two things. First, it is connected to the same aforementioned slime pad, via a sticky piston directly underneath the center of the slime pad. There is a total of 10 full-delay redstone repeaters between the sticky piston and the button, along with 4 redstone. The second thing the button does is trigger a few commands. First, it triggers /setblock 48 59 89 tnt, which places a tnt in the hole of the obsidian configuration above. Second, it triggers /setblock 48 64 86 unpowered_comparator 2 to stop the rain of arrows from the dispenser with the comparator clock. Finally, it triggers /setblock 48 59 88 redstone_block to trigger the tnt. This command block has a redstone torch on it which triggers /setblock 48 59 88 obsidian so that the tnt has no chance of harming any blocks.

The slime pad is a 3X3 pad of slime blocks, resting on top of a sticky piston. If the slime pad is not made out of slime, the contraption does not work.

The Bug:
When an arrow in a slime block is hit by an explosion, its "Motion" is editted.

Data Tag Before:
Motion:[0:0.055203942033460685d,1:-0.6982197271146049d,2:-0.018370680557578734d],damage:2.0d,shake:0b,xTile:49,UUIDLeast:-7537528592826251121L,inGround:1b,pickup:1b,Invulnerable:0b,Air:300s,OnGround:0b,Dimension:0,PortalCooldown:0,Rotation:[0:108.40637f,1:-84.27756f],life:0s,FallDistance:0.0f,inTile:"minecraft:slime",UUIDMost:-4391236948696608803L,inData:0b,Pos:[0:49.140255696017995d,1:57.049799797706285d,2:91.28693671285264d],zTile:91,Fire:0s,yTile:56

Data Tag After 1:
Motion:[0:0.2827444187748325d,1:-1.4244112303550933d,2:0.6303752657987229d],damage:2.0d,shake:0b,xTile:49,UUIDLeast:-7537528592826251121L,inGround:1b,pickup:1b,Invulnerable:0b,Air:300s,OnGround:0b,Dimension:0,PortalCooldown:0,Rotation:[0:108.40637f,1:-84.27756f],life:0s,FallDistance:0.0f,inTile:"minecraft:slime",UUIDMost:-4391236948696608803L,inData:0b,Pos:[0:49.140255696017995d,1:57.049799797706285d,2:91.28693671285264d],zTile:91,Fire:0s,yTile:56

Data Tag After 2:
Motion:[0:0.5137054826918964d,1:-2.1615194785134735d,2:1.2654566425581013d],damage:2.0d,shake:0b,xTile:49,UUIDLeast:-7537528592826251121L,inGround:1b,pickup:1b,Invulnerable:0b,Air:300s,OnGround:0b,Dimension:0,PortalCooldown:0,Rotation:[0:108.40637f,1:-84.27756f],life:0s,FallDistance:0.0f,inTile:"minecraft:slime",UUIDMost:-4391236948696608803L,inData:0b,Pos:[0:49.140255696017995d,1:57.049799797706285d,2:91.28693671285264d],zTile:91,Fire:0s,yTile:56

The Motion appears to drastically change after each explosion, adding approximately 0.25 to "0", subtracting 0.75 from "1", and adding 0.6 to "2".

Steps to Reproduce:
1. Build the contraption above.
2. Load the slime pad and trigger an explosion that will launch the arrows.
3. Load the slime pad again and trigger an explosion without letting the arrows get pushed into the explosion (i.e. disconnect the redstone wire from the start button to the sticky piston with the slime pad).
4. Trigger the explosion and let the arrows be launched into the sky.

Attached Screenshots:
1 Extra Explosion.png

• The results of doing steps 3-4 once.
  2 Extra Explosions.png
• The results of doing steps 3-4 twice.
  3 Extra Explosions.png
• The results of doing steps 3-4 thrice.
  Color Code.png
• Shows what each block/color of wool represents.
• - For example:
• - - Pre-1.9 (Emerald Block) Arrows Launched In 1.8.X (Ignore)
• - - 1.9 - 0&2&3 Tnt (Lime Green Wool) Arrows Launched by 0, 2, and 3 Extra Explosions
  No Extra Explosions.png
• The results of not doing steps 3-4.
  Old-Fashioned Comparator Clock.png
• Used to aid description of first paragraph of the contraption.
  Results Again.png
• Shows results from directly above without Pre-1.9.
  Results Closer.png
• Shows results from directly above with Pre-1.9.
  Results.png
• Shows results. For perspective, each fence is 5 blocks, and each group of 3 fences is ten blocks.
  Spread-Arrow Cannon.png
• Shows the Contraption