Put the summary of the bug you're having here
What I expected to happen was: the blocks would be there and the commands in command blocks would have been there.
What actually happened was: There were no blocks in the world and then when I went into previous versions (14w26b) the command blocks had no commands in.
Steps to Reproduce:
1. Make a world in 14w26b (or lower versions) and then place a command block with a command in it.
2. Go into 14w26c and then everything would have gone.
This does not only duplicate like 100 posts about the issue, it was also posted on the mojang webpage before:
"If you are updating from 14w26a or 14w26b, your saved world has corrupted blocks and it will be heavily damaged once you open it in 14w26c. Please use a world backup that was saved with a pre-14w26 version.
Never use snapshots without first making backups of your saved worlds."
Source: https://mojang.com/2014/06/minecraft-snapshot-14w26a/