The bug

When you stop the respawning of the EnderDragon right after you started it and have an EnderCrystal at the position of a bedrock block, it can cause the bedrock block to drop as an item (see attached video).

How to reproduce

1. Kill the EnderDragon
2. Place an EnderCrystal in the middle part of the portal (fire should replace a bedrock block) Mod edit, 15w51b: You can no longer place end crystals inside bedrock but you can still break bedrock or just predict where the portal will be generated in pre-1.9 worlds.
3. Place three of the four needed EnderCrystals around the portal to respawn the EnderDragon
4. Place the fourth one but destroy it right after you placed it

Eventually the bedrock block will lay there as an item. The explosions can blow it up therefore it is recommended to be in creative mode and float near the middle column during the explosion in order to reproduce the issue.