I was attempting to build an automatic melon farm. When I set up the pistons and torches to pull back a block and push down others, I wanted to test it to make sure it worked. So I destroyed one of the torches so it would flip and it pulled the pistons back but extended them again right away.

Reproduce:
1: Setup can be seen in the video
2: Remove the bottom torch.
3: Notice the strange behavior
4: Can place the torch back to reset the whole system.

Video at http://www.youtube.com/watch?v=AZ2JjBw_2ms&feature=youtu.be
(sorry that it is jumpy and messed up)