Middle mouse clicking a wet tilled dirt block gives you a weird flat dirt panel that is 16x16x1 in your hand and inventory and when you place it, it appears as a purple and black checkered cube which I think indicates a missing texture. When viewed in debug mode it appears that I somehow grabbed a dirt block with a invalid data value of 7. So I guess you need to fix it so that when you grab a wet tilled dirt block it just gives a normal dirt block instead of a dirt block with a data value of 7.

It doesn't happen in 1.7.9, so I'm guessing it's a 1.8 thing.

See attached screenshots.