Placing boats on slime blocks and mounting the boat propels player & boat up hundreds of blocks

Step 1: Place a boat on a slime block. Make sure you are in 15w41a or later.

Step 2: Wait (if you want). The longer you wait, the higher up you go. Maximum height seems to be directly proportional to time spent waiting.

Step 3: Get in the boat. You should be launched upwards tens or hundreds of blocks. When you bounce on the slime block after coming back down, you will bounce up exactly the same height.

Tested to confirm, but original source: https://www.reddit.com/r/Minecraft/comments/3o9rlr/15w41a_boats_slime_blocks_up_hundreds_of_blocks/