Affects Version/s: Minecraft 15w41b, Minecraft 15w42a, Minecraft 15w43a, Minecraft 15w45a, Minecraft 15w46a, Minecraft 15w47a, Minecraft 15w47c, Minecraft 15w49b, Minecraft 15w51b
Fix Version/s: Minecraft 16w04a
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/