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
Linked issues
Comments 6

You do not have to jump on the boat or stand on it. I believe this happens because the boat is acting as if it is falling while on the slime block, and therefore increasing in speed the longer it is there, until you eneter the boat which causes it to bounce on the slime and launch you. This would also explain why the height you gain from launching yourself seems to max out at ~30 blocks because the boat has reached it's terminal velocity.

You do not have to jump on the boat or stand on it. I believe this happens because the boat is acting as if it is falling while on the slime block, and therefore increasing in speed the longer it is there, until you eneter the boat which causes it to bounce on the slime and launch you. This would also explain why the height you gain from launching yourself seems to max out at ~30 blocks because the boat has reached it's terminal velocity.

Thank you for your report!
We're actually already tracking this issue at MCPE-34577, so I will resolve and link this ticket as a duplicate.
That ticket has already been resolved as Won't Fix, which means this is considered a bug but won't be fixed. The description of that ticket or the comments might explain the rationale. Please do not leave a comment on the linked ticket.
Quick Links:
📓 Issue Guidelines – 💬 Community Support – 📧 Customer Support – ✍️ Feedback and Suggestions – 📖 Game Wiki

Thank you for your report!
We're actually already tracking this issue at MCPE-34577, so I will resolve and link this ticket as a duplicate.
That ticket has already been resolved as Won't Fix, which means this is considered a bug but won't be fixed. The description of that ticket or the comments might explain the rationale. Please do not leave a comment on the linked ticket.
Quick Links:
📓 Issue Guidelines – 💬 Community Support – 📧 Customer Support – ✍️ Feedback and Suggestions – 📖 Game Wiki

Thank you for your report!
We're actually already tracking this issue at MCPE-34577, so I will resolve and link this ticket as a duplicate.
That ticket has already been resolved as Won't Fix, which means this is considered a bug but won't be fixed. The description of that ticket or the comments might explain the rationale. Please do not leave a comment on the linked ticket.
Quick Links:
📓 Issue Guidelines – 💬 Community Support – 📧 Customer Support – ✍️ Feedback and Suggestions – 📖 Game Wiki
You do not have to jump on the boat or stand on it. I believe this happens because the boat is acting as if it is falling while on the slime block, and therefore increasing in speed the longer it is there, until you eneter the boat which causes it to bounce on the slime and launch you. This would also explain why the height you gain from launching yourself seems to max out at ~30 blocks because the boat has reached it's terminal velocity.