Whenever you place a boat on a slime block, and then get into the boat, the player and boat will be launched into the air. How long the boat has sat on top of the slime block affects how far it will be launched when the player gets in it, and if the player jumps on top of the boat while setting on slime, it gives it more momentum to shoot up into the air. Max launch height appears to be around 30 blocks. This behavior can also be seen on beds.

How to reproduce:
1. Make a platform of slime blocks
2. Place a boat on top of the slime
3. Jump on top of the boat for 5+ seconds
4. Enter the boat
5. You will then launch into the air. The more you jump, the higher you go.

Expected result:
Boat/player should not launch into the air

Demo video on this bug in action: https://youtu.be/AcUTl8hKU-U?t=314

This bug was also on java back in 2015, as shown by MC-90352