The bug

When running water into string that is attached to a pair of tripwire hooks the string is broken as expected BUT one more than the number broken is dropped.

In other words: if you have two trip wire hooks and between them six string, then seven string are dropped when water runs into and destroys all of the string. However, this behavior (at least in my experience) only happens when the line of string is at least four long.

To duplicate build the device in the screenshots below. (String goes on black wool, and hooks go on block next to and above the yellow wool. I would recommend building everything, putting down the string and then powering the pistons and then putting the water down.

This only works when the water flows in the east-west direction. Testing with the water flowing in the north-south direction will not result in successful reproduction.

Code analysis

Code analysis by pr0cess can be found in this comment.