Sand, Silt and Slush now destroys cobwebs, which broke a lot of contraptions in adventure maps. Although it looks like from my testings, that it only happens when the player goes relatively close into it's vicinity, so it might be possible to fix this, by placing the sand and cobwebs in TEdit much higher up.
I do have to say though, the 1.3.1 update gave a lot of content to help with adventure maps and wiring, but 1.4 is doing the opposite now...