If you place redstone on tall grass, the tall grass will not disappear and place the redstone.
Since the 12w05a Snapshot you could place blocks and replace the tall grass. Redstone does not do this.
*What I expected to happen was...: The redstone would replace the tall grass

*What actually happened was...: Redstone did not place.

Steps to Reproduce:
1. Find a block with tall grass on it
2. Place the redstone on the tall grass.
3. Redstone will not replace the tall grass