The bug

Pressing the "Done" button in the command block GUI resets the success count even when the command was not changed.

How to reproduce

1. Place a command block
2. Enter for example the following command

   /testfor @p
   

3. Run it once
4. Look at its block data. It will have a SuccessCount of non-zero.
5. Open the command block, change some settings if you want, hit "Done"
6. Look at the block data again. It will now have a SuccessCount of 0, even though the command did not change.