This is probably the most ludicrous bug I've ever seen. I was exploring in the ocean and I found a sunken ship. I noticed that it seemed to be cut in half, which isn't too weird on its own - there was an underwater ravine there. The back half and one of the masts was still there, so I looted it and left... and then, further out, I found the front half of the ship...
Some time later I noticed the shadow on the water, too.

How to reproduce:
1. Create a world with this seed: Testificate
2. Head to X 1709, Z -204
3. Now look up.
I'm sorry if it's hard to see, I'm playing in my friend's LAN world and I couldn't wait for day. I'll post a better picture later.