With the implementation of new minecart physics in 14w11 the FurnaceMinecarts would bounce any NormalMinecarts encountered, causing them to accelerate to far beyond the speed of the FurnaceMinecart that pushed it. The NormalMinecarts the slowed, allowing the FurnaceMinecart to catch up and bounce it again.

Dinnerbone tweeted that he was reverting the minecart changes for snapshot 14w17, but the FurnaceMinecarts still behave in the same way, and can cause derailings. This is likely unintended considering the fact that powered rails in 14w17 cannot cause derailings.

What I expected to happen was:
I expected the furnaceminecart to push any minecarts it encountered on the track at a speed similar to it's own speed. (As seen in versions prior to 14w11)

What actually happened was:
The furnaceminecart "bounced" the normalminecart, causing it to accelerate beyond the furnaceminecarts speed. The normalminecart would then slow and get bounced again.

If a "bounce" occurs near a hairpin turn, the acceleration is enough to cause the pushed normalminecart to derail.

Steps to reproduce:
1. Set up a straight line of rail.
2. Place down a furnace minecart and a normal minecart on the rail.
3. Power the furnace minecart so that it collides with the normal minecart. This will cause the normal minecart to "bounce" away.

Steps to reproduce derailing (pictured in attachment):
1. Set up a straight rail with a hairpin turn at one end
2. Place a normal minecart just before the hairpin.
3. Place the furnace minecart and power it so that it pushes the normal minecart around the hairpin. The normal minecart will derail.