From Minecraft Wiki
June 30, 2011
Beta 1.7_01 was released to fix some of the bugs and crashes in Beta 1.7.
2 bugs fixed
- Fixed piston bug. Blocks that used to make the game crash in Beta 1.7 now just get removed by the piston. Worlds crashed are now able to load again.
- Fixed right-clicking on a sheep empty-handed causing game to crash.
- This version's patch number is inconsistent with the other Beta 1.7 updates. The patch number for this version is "_01", while the others are ".n", with n being the patch number. Version "Beta 1.7.1" does not exist.
- This is not the only Beta patch version that does this, with the example of Beta 1.0.2.